In blender or food processor, combine chopped cucumber, scallions, chopped herbs, ginger, garlic, olive oil, and yogurt and process until smooth, about 1 minute. Stir in 1/2 teaspoon salt, pepper, and hot sauce, then transfer to large airtight container and refrigerate 1 hour or up to 4 hours.
In small bowl, stir together shrimp and remaining cucumbers, herbs, and salt.
Fold watermelon or cantaloupe into soup. Divide soup evenly among 4 chilled bowls and top each with dollop of shrimp mixture. Serve immediately.