Combine 2 cups tomato, and 1 cup cucumber, the onion, bread, 2 tablespoons oil, vinegar, garlic, salt, pepper, and 1/2 cup water in a blender or food processor; cover and blend or process until smooth (blend in batches, if necessary). Transfer to a serving bowl. Stir in remaining tomatoes and cucumber; set aside.
Put 1 tablespoon oil in a large skillet over medium-high heat. Add shrimp and/or scallops. Cook, stirring once or twice, until opaque, about 2 to 3 minutes. Season lightly with freshly ground black pepper and add a squeeze of lemon juice, if you like. Serve the gazpacho topped with the seafood. Serve immediately or chill up to 2 hours before serving. Garnish with parsley.