We last had the party looking down into the water off the island, and there's a wreck down below. We may presume they're in a long boat, considering the difficulties of breathing underwater, the dangers of going below with nothing more than a javelin, a dagger and their bare skin. Hopefully, they're also reassured that the ship holds promise, as well.
At this point, the party can very easily spoil all our plans by simply saying, 'Nah.' And that is the important thing about sandboxes - that this won't be the last dangerous opportunity and this won't be the end all and be all of the treasure that's in the world. There's no amulet, no magic helmet, no critical carving waiting in the ship that must be recovered or the world will come to an end.
I can only recommend that DMs stay away from that kind of scenario. End of the world set-ups, where the balance of fate depends on the party's actions, are great things for films that last two hours, but they are tiresome annoyances when strung out over a period of three or four runnings. Parties will ask - and not kindly - why it is always up to them to do everyone's dirty work?
Worse, it's painfully obvious where this dirty work is coming from. Here's an idea. Give the party a break. Let some NPC be the world's last hope and let the party dig around for some treasure.
Let's consider this ship. How much work am I going to do before the party goes down there?
Well, the only reason why I need an actual map of the ship would be if I planned for a fight to happen. If a fight took place on the deck of the ship, then I would need the metrics for where everyone would be standing when swinging or being swung at. But here's the thing: this is an underwater adventure. I can easily move the combat twenty feet to the right or left of the ship, above the soft sand of the shelf where the ship is sitting. As well, none of the party will be standing on a surface. They'll be floating above it - so the surface doesn't matter. All I really need for a combat map is a blank hex map, painted blue. I can make this in about two minutes during the actual run, so no point in making it ahead of time. If the party changes their mind and doesn't dive down, I've lost nothing.
I know that most DMs would view the situation - extended underwater movement - as a sign that something ought to be found on the island to make it easier for the party. The captain has a set of convenient bracers of underwater action or a cache of water breathing potions were found by the party just two sessions ago (with the expectation that they would come to this island and see this ship). I loathe, hate and despise this sort of shit. I know the DM is very proud of their ability to think ahead; "Aha, I am so clever, I will give these scrolls of water breathing, then they will go to the island and . . ."
Except that parties are not dumb. Here are four happenstance suppositories of water breathing, oh so ready for use! You can physically feel the cars being attached to the engine for the next part of the journey. Isn't that nice. The party knows the DM has planned this ship and probably has a 3-D diagram ready - can we feel the guilt yet? How about the responsibility for not disappointing the DM here. Don't tell me it's not there. Parties may drag their feet, but they get on the DM's train and go anyway. "Well, shit . . . he's done all this stuff of us. I guess we can go on his adventure, since he's made it already."
It is this preparation mind-set. Granted, it's nice that the DM works on their campaign and has a neat ship graphed out for the party's enjoyment once they descend - but this insistence that the party must have the most obvious and difficult obstacle removed from the start is crippling to the game. The underwater adventure is interesting because it is underwater. If everyone has the means to make this submerged nightmare like a walk in the park, where's the challenge?
If you've played for years, you've run into it time and time again. Going to fight a lot of undead? Well, I've got special holy symbols for everyone. The castle is at the top of an impossible cliff? Here, you'll need these remarkable boots of floating and wafting. Oh, you have to adventure in the Forest of Always Night? Just a minute, I've got four sets of goggles in this trunk here. No matter what the environment, helpful NPCs are soon tripping over themselves to get you whatever special gear you'll need to handle it. No reason why you should just trust to innovation, determination and luck!
If we put the ship 18 feet underwater, that's deep enough. The party doesn't need water breathing. They just need to strip down, dive 18 feet, do what they can for three rounds and then return to the surface again. Yes, it's hard. Yes, it's worse if someone gets trapped down there. But watch what a party does or tries to do to get around this without straight on magic. Watch them pull out their spells and cantrips and try to figure out shortcuts, temporary alternatives and plans. Watch their faces when you tell them, "Yeah, sure, that could give you another couple of rounds."
Yes! The player smiles proudly, looks around at the other party members, gets a few compliments and the party thinks, "Fuck, we're rocking this!" And what is it that makes them think they're rocking it? They've figured out a way they can stay down for five rounds rather than three.
Let's not deny the party their small victories. Let's not ruin their adventures by making their lives easy for them. Let's try them at having a combat where they can only engage while wallowing about on the surface - it only means that every hit will be ten times as meaningful.
I can't understand why DMs do not see that.