his nails could get "painted" when he goes in warm or cool water

That's genius, happy happy genius. And their castaway island can have a cave on it for all their shelter and cave!fic needs.

(If you've seen the mighty boosh episode where they're on a desert island, I can sort of imagine Peter and Jared getting that kind of crazy after a while and making juries and judges out of coconuts and having full trials about who's fault everything is and why.)