On a work surface, finely chop enough of the watermelon and cucumbers to yield 1 cup each; in a medium bowl, combine with the quartered tomatoes. Using a food processor and working in batches, puree the remaining watermelon, cucumber and tomatoes, the bell pepper, onion, vinegar and 2 tablespoons olive oil; season with 1 1/2 teaspoons salt and 1 teaspoon pepper. Refrigerate for 20 minutes.
In a large bowl, toss the shrimp with the remaining 1 tablespoon olive oil and season with salt and pepper. Heat a large skillet over medium-high heat, add the shrimp and cook, turning once, until opaque, 3 to 4 minutes.
Divide the gazpacho among 4 bowls and top with the reserved chopped watermelon mixture, the shrimp and feta.