Place the cold watermelon cubes and cucumber half-moons in a large, wide serving bowl. If the watermelon is very juicy, drain the cubes for 5 minutes first.
Whisk the lime juice, lime zest, olive oil, honey, and black pepper in a small bowl until blended.
Add the red onion, chopped mint, and about three-quarters of the feta to the watermelon and cucumber.
Pour the dressing over the salad. Use two large spoons to lift and turn the ingredients gently until lightly coated.
Top with the remaining feta and extra mint. Taste, add a small pinch of salt only if needed, and serve immediately while chilled.