Besides the general injunction against eating large reef predators, it's possible to buy ciguatera test kits you can use on the raw fish.
Methods vary. Some types of fish are amenable to being sushi...others not at all. Lime marinades/ceviche are a good way to "cook" without heat, and the on deck solar cookers
are alleged to work well and to keep heat and fish smell out of the cabin
Some cruisers find they are happier with deck
or rail-mounted hibachis that they fuel
with either charcoal or local wood, sufficiently dried. This is perhaps possible on remote
islands, but I suspect foraging for the barbeque is discouraged in some places.