In a small mixing bowl, whisk together the olive oil and lime juice. Set it aside.
Using a large mixing bowl, add in the cubed watermelon, cubed English cucumber, and mint leaves.
Drizzle the dressing over the salad. Carefully toss the dressing and salad together. (Do this as gently as possible, the watermelon and cucumber will release a lot of juice.)
Sprinkle the feta cheese over the top of the salad and serve immediately.