Hey! Thanks for the comment. I'm real slow to respond to things sometimes haha. I tried to hint at a few ways they could achieve this and I'm partial to letting the PCs coming up with their own creative solutions as well.
***Spoilers Incoming***
My thought was that one could befriend a pterodactyl after their transformation and hitch a ride to the mainland ( or sacrifice themselves to save the party).
Also the bell tower has "plentiful rope" that could be used to hoist the sunken ship up as long as they convince (or exploit the strength of) a T-Rex for leverage. I'd also rule that if the crew of prisoners is cured of their scurvy they could work together to hoist out the ship. Of course, there's also the control water scroll. The only downside is that the Plesiosaurus will be terrible sad.
If all else fails, if they survive the storm they can chop down some trees and make some rafts haha.