In a large mixing bowl, whisk together the olive oil, vinegar and salt. Add the watermelon, cucumber, feta, and basil. Toss gently to coat evenly with the dressing. Transfer to a serving bowl and sprinkle with the chopped mint and sea salt. Serve immediately.
3 Tablespoons extra-virgin olive oil, 2 Tablespoons red wine vinegar, ½ teaspoon kosher salt, 4 cups seedless watermelon, cut into 1-inch cubes, ½ cup cucumber, seeded and cut into ½-inch pieces, ½ cup feta cheese, crumbled, ¼ cup fresh basil, roughly chopped, ¼ cup chopped mint, 2 teaspoons flaky sea salt