Sprinkle 1/4 teaspoon pepper and 1/4 teaspoon salt over scallops and cucumber. Arrange in a single layer on a grill rack coated with cooking spray. Grill 3 minutes on each side or until scallops are done and cucumber is well marked.
Remove from heat; cut cucumber into 1/4-inch slices.
Combine remaining 1/8 teaspoon salt, juice, and oil in a large bowl; stir with a whisk.
Add cucumber, lettuce, watermelon, and mint; toss gently to coat. Divide the watermelon mixture evenly among 4 plates. Top each serving with 3 scallops and 2 avocado slices.
Sprinkle evenly with remaining 1/4 teaspoon freshly ground black pepper.
Sustainable Choice: All scallops, whether farmed, diver-caught, or wild-caught, are great options.
