In a small bowl, whisk together the olive oil, red wine vinegar, and 1/2 teaspoon of kosher salt. Set aside.
In a large mixing bowl, combine the watermelon, cucumber, feta, mint, and red onion. Drizzle the prepared dressing over the mixture and gently toss until everything is well-coated.
For a final touch, sprinkle more mint on top and, if desired, add a pinch of flaky sea salt. Serve immediately.