Cut 8 cups watermelon into 1-inch cubes and place in a large serving bowl. If using, add the sliced cucumber and red onion.
In a small bowl, whisk together 3 tablespoons olive oil, 2 tablespoons lime juice, 1 tablespoon honey, 1/2 teaspoon salt, and 1/4 teaspoon pepper until well combined.
Add 1 cup feta cheese and 1/2 cup basil (plus 1/4 cup mint if using) to the watermelon.
Pour the dressing over the salad and toss gently to combine. Be careful not to over-mix.
Drizzle with balsamic glaze if desired and serve immediately. Let sit for 5 minutes before serving for best flavor.