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.