Rinse and dry the fish thoroughly. Whisk together the egg whites and water and combine with the salt in a large mixing bowl. Work the mixture with your hands to distribute the water and egg whites in the salt. The salt mixture should hold together when you press a mound of it together between the palms of your hands. If it doesn't, work in another egg white.
Fill a baking dish that is 2 inches longer than the fish about 1/3 full of the salt mixture. Place the fish on top. Cover the fish with the rest of the salt mixture and smooth the salt over with a spatula. Place the fish in the oven for about 40 minutes. The only way to know whether the fish is done is by inserting an instant-read thermometer through the layer of salt and measuring the temperature of the fish. When it reaches 135 degrees F, take it out, and serve immediately.
Bring the fish to the table on a baking sheet or on a large platter and crack it with a mallet or even a hammer. Pull back the salt layer, then peel off the skin with a fork. Serve on hot plates.