Poaching is an excellent way to preserve the fish's moisture, particularly if you're serving it cold. If you poach fish frequently, then it's worth investing in a fish poacher, but you can improvise with a large deep pan, such as a turkey roaster, and a meat rack.
Poach the fish either in salted water or in a flavored broth. Either way, preheat the liquid. Wrap the fish in cheesecloth, place it on the rack, and lower it into the liquid. The liquid should barely cover the fish and be kept at a gentle boil.
Poach the fish about ten minutes per pound, but check after eight minutes (per pound) so that you don't overcook it. Slightly undercook the fish if you're planning to cool it in the liquid, because it will continue to cook as the liquid cools.
From the "Legal Sea Foods Cookbook" by Roger Berkowitz and Jane Doerfer, illustrated by Edward Koren.