Mustadio - he's an engineer, he could probably build a boat with the proper materials.

Terra - good magic and sword skills, plus she can always morph and fly out to get help, or, at the very least, pull down Fire 3 from the sky to attract attention.

Faris - Something tells me a pirate would be helpful on a desert island. Especially if it gets visited by other pirates. Of course, she might not let us set fire to the rum, but I think Terra might persuade her otherwise. And her job versatility doesn't hurt either. And if we ever make a boat, we do need someone who can steer it, as well as someone who knows how it should be built. Who better than Faris?

Edgar - As a man of tools, he'd probably be able to help Mustadio with that boat.