The best way to thaw frozen fish is to plan ahead and defrost it overnight in the refrigerator. Although there are faster methods available, this one best prevents loss of moisture.
Seafood needs special care because it can spoil so quickly, and thawing at room temperature or in warm water can cause bacteria to multiply rapidly. If you do not have enough time to thaw fish in the refrigerator, a faster way to do it is by placing the fish in a sealed plastic bag under cold running water for about an hour.
Alternatively, you can thaw frozen seafood in a microwave oven. Use the lowest setting available and stop before the fish starts cooking. It should still be icy, but pliable. Use a microwave only if you plan to cook the fish immediately.