My underwater campaign was suspended for a session as two of the players were getting married to one another, while four of the remaining players were taking part in the wedding party and the DM was giving the bride away. As such, I was granted a reprieve on inventing rules for fighting and moving underwater ~ but as the campaign is picking up this Saturday, it wouldn't hurt to embark on the subject for a few days.
First of all, fighting. I had scattered thoughts about this, wondering if I should come up with a different actual combat system strictly for underwater combat [yes, my mind is stupid enough to go there] . . . but sanity and time constraints imposed their grip upon me and I have moved on. At the moment, I am entertaining a practical, yet punishing alternative that I am certain the players will hate while at the same time possessing a wisdom that my players will accept. I'll try to explain it simply . . . but please remember that it is always much harder to explain a rule in print to people who do not play the game than it is to present it in a campaign, person-to-person.
The argument goes like this: because land-dwelling players are thoroughly unfamiliar with fighting underwater, it follows that first efforts will fail in the extreme. While able to water breathe and perhaps even use features such as the ocean/sea/lake bottom or rock outcroppings for support, they will underestimate the resistance and nature of the water, their vision will be distorted by the changing light underwater, they will be unbalanced by their boyancy and so on.
To reflect this, the combat effectiveness of a Ninth level fighter should be reduced to that of a 1st level fighter in ordinary conditions . . . but temporarily. Initially, I would impose a -8 penalty to hit against all characters, regardless of class. This penalty would then be reduced by 1 per four [possibly eight] successful swings ~ but please note, I do not say "hit."
A successful swing is any attack that would have hit if the circumstances would be completely normal. Many successful swings would not, however, result in a hit. I will give an example.
A 7th level fighter, Garth, who has never swung a weapon underwater is using a trident against a giant crab, armor class 3. Garth normally has a THAC0 of 14 ~ at the moment, however, Garth's thaco is 22 (incorporating the -8 to hit). To hit the crab and cause damage, Garth will need to roll a 19 or better . . . but to score a successful swing, Garth will only need to roll an 11, this being the number a 7th level fighter would normally need to hit AC 3.
Okay, how am I defining a "successful swing"?
I'm calling it any roll that gives credence to the idea that Garth has failed to hit but has comprehended why the hit failed! That is, Garth has tried, sees the path to improvement and thus gained insight on how to do it better in the future. Every four times this happens (including any successful swings that happen to also be hits), Garth improves, reducing his negative modifier by 1.
Thus, steadily, the more time Garth spends fighting underwater, the better he gets and the closer his underwater combat skill matches his ordinary combat skill. Once both have lined up, then it is presumed that all needed changes and adjustments for fighting underwater have been overcome by Garth and he has fully adjusted. In all, assuming four swings per modifier, Garth needs to succeed 32 times.
This is much, much easier for a 7th level fighter than it would be for a 1st level of anything, particularly a mage or a thief. Racking up 32 hits for a high level might take a few combats; accomplishing the same at low level would be a very difficult slog, made worse by having to put oneself in danger for quite a while without ever having even a chance to cause actual damage.
Consider: a 1st level mage in my world has a normal THAC0 of 21. Underwater, to start, this becomes a 29. To get a successful swing on the crab would require an 18 on a d20; to do damage would require a 26. This means the mage has to put himself in the way of the crab's claws (and anything else) for a really long time, getting snapped at and punched around without any hope of helping kill the beast.
Three points, however. First, I wouldn't expect many first levels to do any underwater adventuring; it is difficult to manage, what with water breathing and having the means to truly be equipped for the tasks involved. The party I have going in is largely 7th level or better, with 5th to 8th level henchmen for support. There are a few players who have low level characters, as they haven't been playing in the campaign all that long, for whom this is going to be fretful; but they have considerable support to protect them while they get bashed around. There is one 1st level fighter who did join just two sessions ago . . . but lucky for him, he was incredibly lucky in his rolls and wound up with an 18/00 strength, with a natural +3 to hit. That will help a bit.
Second, I did not intend to drop them into the heart of the adventure right off. I expect them to wander through some undersea "wilderness" for a bit as they approach the main adventure, so they will have a chance to cut their teeth on some smaller beasties, roaming about the sea. This should give them a chance to prepare themselves . . . and if some decide not to prepare themselves, letting the big tough party members handle things, they'll certainly regret that later!
Finally, my experience system rewards players for getting bashed around. So at least they won't be standing up to sea creatures for nothing. And they'll be taking a hit for the team.