I am currently building a campaign for DnD.
The main plot plays out on an island the heroes are stranded after their ship goes off course and sinks.
They discover that this island is ruled by monsters that serve a mighty demon/ancient evil. There are no humans beside a small town/city that is regularly visited by the demons to take a few of the people living there to perform a ritual, sacrifice those people, feed on them, etc.
The main goal for the group is to defeat the evil force and free those humans.
The island is undiscovered because it is so far from the other continents.
My problem to solve
How do I stop my characters from just turning around and leaving the island in a self built ship/raft?
My first idea was to make it geographically difficult/impossible to leave by surrounding this island by a nearly closed ring of mountains (kinda in a U shape). An explanation for this could be that the island used to be a volcano that exploded (a real thing) and the mountain ring is what is left of the volcano.
My problem with this idea: is it believable? Isn't it a bit too random for them to have been stranded exactly through the passage that is open?