This is very a light, fresh, and cool salad that can be enjoyed anytime and with any meal or just as a nice in-between meal snack.
In a bowl, mix vinegar, sugar, cilantro, and red pepper flakes and set aside.
In another bowl, combine watermelon, cucumbers, and onions. Pour dressing over fruit and vegetables and toss to coat. Chill in the refrigerator for at least 1 hour.