Soften the gelatin in the cold water. Bring tomatoes to a boil, reduce heat, an d slowly whisk in gelatin water mixture. Add cream cheese and stir until melted . Remove from heat and cool to room temperature. Whisk in mayonnaise, onion, ce lery, salt, Tabasco, and lemon juice. Stir to blend completely. Refrigerate for about 15 minutes, then fold in crab and shrimp. Transfer mixture to a serving bowl or decorate mold and refrigerate overnight. A fish mold is ideal for this mousse. For decoration, use olives for the eyes, fresh parsley or fresh dillwe ed for eyebrows, sliced almonds for scales, and pimiento strips on the tailfin. Serve with cracker, cocktail toasts, or with slices of dark rye or pumpernickel .