Shell and devein shrimp, placing only shells in large saucepan. Rinse shrimp; drain.
Pour 3 cups water over shells; bring to boil. Reduce heat; simmer, uncovered, 20 to 25 minutes, or until liquid is reduced in half.
Strain liquid; discard shells and return stock to saucepan. Stir in tomatoes, corn, less sodium soy sauce, sherry and lemon juice; cover and bring to boil. Reduce heat; simmer 5 minutes.
Add shrimp and scallops; simmer, uncovered, 2 minutes, stirring occasionally to break up tomatoes.
Blend cornstarch and 1 Tbsp. water; stir into soup. Bring to boil; cook 1 minute, stirring constantly. Stir in green onion and pepper; serve immediately.