Preheat the oven to 350 degrees F. Butter 8 (4-ounce) ramekins or baking dishes, and set aside.
In a food processor, puree 1/2 of the crawfish and all of the halibut. With the machine running, add the salt, white pepper, cayenne, and garlic, and with the machine running, add the cream and white wine. Add the egg whites. Transfer to a bowl and fold in the green onions, dill, and remaining crawfish tails.
Put the ramekins in a large roasting pan, and pour hot water into the pan to come halfway up the sides. Bake until the custards are just set and puffed, and lightly golden brown on top, 15 to 20 minutes. Remove from the oven and let cool. Cover with plastic wrap and refrigerate at least 3 hours and up to a day.
Arrange the watercress around the rims of 8 small plates. Remove the molds from the refrigerator and carefully run a thin, sharp knife around each mousse. Shake gently and unmold onto the plates. Serve cold with Homemade Herb Mayonnaise, and small French bread croutons.
Yield: 8 servings