Without a scuba mask to help the eye see (we're designed to see in the air, not underwater), focusing underwater produces an extremely blurred image, even up close. This, like breathing, has to be addressed if an air-breathing humanoid is expected to function under water. It seems to me that any spell that transforms the breathing capacity of a character to be able to function under water must also include a feature that transforms the eye so that it will pick up images underwater in a way that the human eye does not - in effect, giving us the vision we would have if we were wearing a diving mask.
Still, since the transformation is done by spell rather than by equipment, we're free to assign any amount of visual ability that suits us or the campaign. I choose to argue that objects and moving creatures become blurry after a distance of 20 feet, or four combat hexes in my world, and indistinct (that is, one thing cannot be separated from its environment or effectively from other similar things, moving or not) after a distance of 8 combat hexes.
This means that even if a spell does have a range higher than 40 feet, and even if a weapon can be thrown farther than 40 feet, nothing can be targeted at all beyond that distance. Under the water is a close-up world. Get used to it.
Like the eyes, a water breathing spell must also include an underwater speaking component - otherwise, any adventure under the water is going to quickly get boring, what with everyone endlessly making hand gestures at each other. Whatever we might try to do, without getting ludicrously dictatorial, characters are going to say out loud what they're doing and what they want others to do even if we try to stop them ~ so let's not bother. Let's just assume that they, and the intelligent creatures they encounter, can speak underwater normally . . . the campaign will simply be more interesting that way. Others can prefer to mime their hours away but by a small adjustment to the magic involved I can skip it, so I will.
The distance sound can travel is another thing. Arguably, water amplifies sound better than air for a variety of creatures ~ why not suppose that the "speaking" component of the water breathing spell works in reverse to the effects of vision? We can say that speaking distance is doubled, so that rather than limiting relatively intelligible speech to sixty feet (12 hexes), we can increase that to 120 feet. We may not see the enemy approaching but we can hear them . . . and prepare ourselves accordingly.
This is tricky. I don't want to start from scratch designing movement rules for what might be the only underwater adventure I ever run - it would be easier to make use of existing rules. Some might remember that I use a series of action points, or AP, that serve as a measure of how much activity a given creature can manage in the time period of one round.
Because of water's liquidity, I prefer to argue that all movement is much slowed. The easiest way to express this slowing is simply to half all movement (that is, halving all AP/round), arguing that a character has to move by kicking their feet, pulling themselves forward with their hands, pushing away and various other swimming movements that must often be performed with one hand if the character happens to be holding a weapon. It would be painfully complicated and aggravating to try to assign a measure to each possible example of such movement . . . thus it is much, much easier to simply cut the AP in half and have done with it.
A normal human, unarmored and with a minimum of equipment, normally has an AP of 5. Half of this would incorporate a fraction and that, too, is irritating. Alternately, we might leave the players' AP unchanged and simply double the costs of everything: it would, for example, require 4 AP to swing a weapon underwater instead of the usual 2 upon the surface.
[I trust the gentle reader will follow the links given so that they can understand this argument. I trust my readers are diligent about all their research, nyet?]
Either way, halving the character's AP will produce fractions, which are irritating; doubling the AP costs will make many things too costly to manage in the space of one round, requiring people to keep track of how much time they spent towards what ~ and this is also irritating.
Suppose instead that we simply count 2 rounds as 1 round for the sake of resolving combat. Whereas the combatants normally take their move in a 12-second period, followed by the enemy's move, suppose we just say this happens over twice the time period? This lets us keep the AP and their costs exactly as they are, likewise slowing down the enemy underwater (even if they're crabs, fish, humanoids, whatever). If something seems especially deserving of being faster than that (say, a barracuda), we can simply double their potential attacks in one "round" (or UR, for underwater round) to balance out their particular threat.
Underwater Rounds & Magic
Of course, being underwater shouldn't affect the speed with which magic is cast and discharged (see my Spellcasting rules, which are designed to limit the power of mages and clerics to deliver rapid-fire spells). To my mind, a floating, concentrating mage, performing all the usual verbal and somatic cues (I don't use spell components), can do so as easily underwater as anywhere, once they can breathe, see and hear in that environment. The delivery of magic, too, is subject to AP ~ but without the slowing down of movement necessitated by swimming and twisting, it can be cast and discharged in normal time. That is, in the two-round period of the UR, twice as fast.
Normally, a mage casting a 1st or 2nd spell in my world would have to spend a round casting, then wait until the next round before discharging the spell. A 3rd or 4th level spell requires two rounds of casting before yet a third round is needed to discharge the spell. A 5th or 6th level spell requires three rounds of casting; and so on. This profoundly limits mages in how fast they can hit the enemy with spells ~ much reducing the power of a mage with a big pile of spells and balancing the game much more in favor of those who do actual fighting.
Underwater, however, the mages can cast that 1st/2nd level spell and discharge it in the space of one UR. Effectively, given that everyone else is slowed by water, mages will seem as though they are hasted (or, as far as every other campaign in the world goes, casting spells normally. Hah. But seriously, try adjusting the spellcasters in your campaign to my system if you feel they're walking all over the enemy far too much!).
More about magic below. I want to stay in the realm of movement a bit longer.
Lateral & Vertical Movement
Once again, I want simple rules for moving around in an environment that permits up and down movement ~ meaning that I want to consider only minimal conditions for limiting the movement of combatants in an environment where it is possible to move vertically as well as horizontally.
For someone who represents combat as a tactical representation on a map, the 3-D problem is a big issue ~ that is, showing people at various heights above the ocean/sea/lake bottom clearly and easily enough that a combat map can be grasped by the participants. For this I plan to define all vertical movement as "ascent" [in a desperate attempt to avoid the much-overused word "level"], which can be measured in numbers from the bottom up. Thus, if we are floating at 2-ascent from the bottom, we are in an imaginary 2nd vertical hex above the bottom, or a movement of 1 AP (remembering we are talking underwater rounds) from the hex that meant we were standing on the bottom.
So, if a player chooses to move three hexes, they would declare both their lateral movement and their vertical movement thus: "I move 2 hexes forward and ascend 1 hex." A number is then added to the side of the character to show what their ascent is, thusly:
Lateral movement would then be demonstrated on a typical hex map.
All distances (and therefore potential for melee) would be measured by the square of the hypoteneuse (which can be calculated roughly in our heads or exactly through a quick-and-easy prepared excel sheet). If one combatant, then, was 1 hex away from an opponent laterally and 1 hex away vertically, the combatants would be judged to be 1.4 hexes apart and therefore not close enough for direct melee. Combatants would need to be directly under or over their opponents or have the same ascent in order to actually fight hand-to-hand.
I have noticed that the original monster manual gives various weapons to marine humanoids, including most often the trident and the spear, but also include the dagger, javelin, net lance, sword and crossbow. I have to wonder what reflexive animal sinew is used for the string of the last, as well as the means of forging metal to create the firing mechanism underwater; clearly, the crossbow is a placeholder for some kind of harpoon gun . . . perhaps there is some means of an intelligent race creating such a thing, so I'm prepared to let it stand. A bigger problem, I think, is the balance a sword needs to be effective vs. its use underwater and obviously the difficulty of throwing or swinging a net in a liquid environment. The latter sounds appropriate, as nets have always been used in association with tridents, but not actually in the water.
Spears, tridents, javelins and harpoon guns normally incorporate wood, so we have to wonder how these weapons are made in an environment without any. At least wood is denser than water when left in water long enough, but that doesn't explain the absence of available wood for working (unless we want to argue an entire culture is armed with drift wood that has fallen into the sea). Bone seems much more practical; it is denser than water when wet (so it won't float), less likely to rot and ~ given the size of the beasties roaming an underwater sea world ~ more likely of the right flexibility, length and shape to make spear/trident handles from as well as javelins. I feel that an underwater "sword" would be more like a macuahuitl in design, incorporating various stones or possibly ivory teeth.
Beyond the above, there aren't many weapons to choose from. A bo staff, used correctly, should be perfectly effective underwater (it is the drawing back and stabbing motion with the staff that is needed, not the swinging motion). A quarterstaff would be too thick and long, and difficult to make of bone.
Virtually every other weapon is either too thick to use in water or has a balance that simply couldn't be mastered if the combatant were not standing on a fixed, gravitationally supported surface. The heavy balance of a sword, axe, hammer or pole-arm is deliberately outward from the body, with the wrist being used as a fulcrum, in order to obtain the best possible newtons of force at the long end of the swing (which works very well), but underwater this just spins the combatant around, swinging them off their feet. The necessary motion must be a stab, not a swing - which makes a foil a more effective sword than a sword; something like the macuahuitl would be effective only if the combatant were astride a heavy steed (a one-ton hippocampus, say), the mass of which would counteract the force of the swing, or were otherwise fixed to the ground.
In my normal combat system, it does not require an AP to turn around in a hex; each combatant has a facing, but adjusting this facing without actually moving is not considered a penalty because I believe that any combatant must be doing this all the time, automatically, when fighting multiple opponents.
Underwater, however, turning 180 degrees requires a little bit of swimming, so I would say changing facing more than 1 hex would be a 1 AP penalty.
Regarding swimming, which is a sage ability in my game, I feel the best adjustment for possessing this skill would be to add 1 AP to any combatant possessing the ability. I had considered adding 2, but knowing the system I use as well as I do, I know that even 1 AP will be a considerable advantage; I don't know that I want to give double that advantage. I'd like to play test underwater combat for awhile ~ if it seems the advantage of 1 AP isn't enough, I'll improve it.
Normally, combatants above opponents gain a +1 to hit while those below suffer a -1 penalty. I feel that this should be reversed underwater for two reasons. First, the light is above the combat, so the combatant underneath will see the outline of the target more clearly than the combatant looking down into the gloom. Secondly, it is easier to swim upwards and away from the pressure of water than it is to swim downwards; thus upward movement is always easier and therefore more deserving of the bonus.
Sinking & Floating
Both of these are troublesome, as they tie up a lot of attention in the middle of a combat, particularly if we are talking about more than 10 combatants. I expect I will have at least one massive combat during this adventure with up to 100 combatants, so I want to keep the system as simple as possible.
Rather than argue that all surface humanoids will automatically float if they are without ballast and will automatically sink if they have ballast, I'm going to argue that characters must necessarily adjust their encumbrance until they have achieved a mid-point, where floating and sinking doesn't happen. IF a character insists on going naked, without weapons, then they will float; and if a character insists on carrying more than 1/5th of their body weight in equipment, or wearing clothes that do not permit easy swimming, then they will sink. Ad hoc (and not because I think it is accurate, but because it is practical), I will argue that a character able to float will rise 1-ascent every other round, while a character that sinks will drop 1-ascent every round.
Simplicity is the key here.
Here is my initial list of banned spells underwater. I expect this list to be expanded and adjusted (players may argue for their partial inclusion), particularly in regards to spells that will "sort of work" but not fully. The work is going to be so pervasive that it is just easier for the campaign to make rulings as we go (discussing the final decision with the players and coming to a consensus) and then updating the wiki during the game.
Cleric: lower water, flame strike, insect plague, control weather, wind walk.
Druid: predict weather, fire trap, create wildfire, call lightning, flame blade, pyrotechnics, summon insects, call woodland beings, control winds, wall of fire, insect plague, conjure fire elemental, fire seeds, control weather, creeping doom, fire storm.
Mage: affect normal fires, burning hands, find familiar, pyrotechnics, flame arrow, fly, gust of wind, fire charm, fire trap, wall of fire, control weather, ice storm, lower water, incendiary cloud, meteor swarm.
Illusionist: wall of fog.
Some will argue that fly should work, but I'm going to be clear on this. "Fly" makes the body light enough to ascend; I do not consider it anti-gravitational. Therefore, I would accept that a caster could cast fly, but it would quickly cause the caster to zip vertically upwards and out of the water at maximum speed, until completely entering the atmosphere (of course, this could be a good thing!).
I do not equate magical lightning with real lightning, therefore a magical lightning bolt will work underwater exactly as a magical fork of power. Likewise, fireball does not create real fire, so it will also work underwater; the effect is an massive increase in heat in the area of the spell that lasts an instant, causing damage by cell rupturing. The "fireball" component is the visual shock of the air turning to a blinding brightness than instantly disappearing again ~ not the creation of actual fire.
I do not remotely hope that this accounts for everything. Actual play will reveal hundreds of issues that I can't predict ~ the key will be to keep updating the wiki as we move forward and adjust rules as it becomes necessary. I'm going to treat this adventure as an experiment, being open to arguments ~ because that is the only rational way to solve the issues involved.