Combine marinade ingredients in a bowl. Add fish, shrimp and scallops; stir to coat. Let stand 10 minutes.
Bring stock and winter melon to a boil in a pot; reduce heat to simmer.
Stir in seafood; cook 2 minutes. Add ginger, green onion, mushrooms, bamboo shoots, peas, wine and white pepper; cook 1 minute. Add cornstarch solution; cook, stirring, until soup boils and thickens. Turn off heat. Add egg whites, stirring until they form threads. Serve in individual soup bowls.