Make the dressing: In a small bowl, whisk together the basil olive oil, watermelon balsamic, lime juice, garlic and salt.
Arrange the watermelon, cucumber, and red onions on a large plate or platter. Drizzle with half the dressing. Top with the feta, avocado, mint, and serrano pepper, if using, and drizzle with remaining dressing. Season to taste and serve.
