Building a ship to escape can be a secondary quest as enjoyable as the main one. It's not an easy thing to do - and the demons may not allow it.

Though not a full duplicate, you may want to take a look at this question and more specific this answer: worldbuilding.stackexchange.com/questions/83317/…

Take a look at the reboot of the Tomb Raider games. Take a look at Lost. Take a look at Cast Away. Or simply say: Yeah, the Demon watches your feeble attempts and laughs demonically, when he uses his dark magic to smash your raft. Or he sends his minions in order destroy your boat. Or whatever. It's D&D, it doesn't have to be realistic. In fact, something realistic would prove a problem even after the demon is done in.

While this is (as a Worldbuilding question) on-topic here, you could also look through Role-playing Games for more suggestion

6 answers and no upvotes for Nautha! If it is good enough to answer it is good enough to upvote. Get Nautha some rep for this cool scenario.

Making it the demon's fault has the added advantage that it motivates the players to off him.

You might not even need to do anything if your players never consider that option to begin with.

Gratuitous The Exterminating Angel reference.

building a raft and building a ship capable of long distance travel on hte open ocean are very different things. you are talking about months of work.

Building a ship is hard enough (and long, and expensive), sailing it home when you don't know where you are in the first place is very hard

@njzk2 but it still gives the players the feeling they might make it if they just try hard enough.

@Nautha If you're the DM, you actually give that impression. I think it's more a question of how to set up the game and the players' expectations. Having a reason why they can't leave is neat, but ultimately if they don't want to complete the quest and kill the demon, there is a larger problem.

Were your players the crew of the ship or passengers? If they were passengers and not sailors they most likely have no idea how to build an ocean sailing ship nor how to navigate.

"The island is undiscovered because it is so far from the shore of all the other continents." -> If it's so far away, how did your players manage to get there? If it was possible to sail there, it would have been discovered by now. If it's not possible to sail there, it's also not possible to sail back from there.

A massive weather balloon masquerading as a guardian, obviously. You kids....

No mention of Ys I: Ancient Ys Vanished? That game takes place on an island that has been cut off from outside contact for ages due to a constant storm surrounding it that ships can't cross without severe damage (probably sinking them, and definitely sinking a raft)

@Nautha you could let them try. How do they buy timber? Do they know anything about ship-building? Even if they get through all that, once they build their ship, and they try to sail off, a storm can easily take care of the ship, and throw them back to the shore. I think by just making sure they consider the logistics involved in building a sea-worthy ship that is actually capable to bring them home, and then stocking it with the necessary supply should be enough to convince any player that it's not going to work.

Building it wouldn't be that hard. Designing a seaworthy vessel, however, is quite another matter. I would say it's at least DC 25--and they have nobody to learn from and thus no way to take any ranks.

How mighty is that mighty demon? In 3.5, for example, having some casters of, say, level 9 or so, can get you out of almost anywhere, so you would have to have some positive motivation for a party more experienced than that - like a being of greater good asking nicely, riches to be had from the demon, etcetera.