Cut the watermelon into evenly sized cubes, approximately ¾-inch each. For best results, chill the watermelon for about 30 minutes before cutting.
Peel the cucumbers if desired (English cucumbers can be left unpeeled). Slice in half lengthwise and scoop out seeds if necessary. Dice into pieces similar in size to the watermelon cubes.
In a small bowl, whisk together the olive oil, lime juice, honey, salt, and pepper until well combined to create the dressing.
In a large bowl, gently combine the watermelon, cucumber, mint, and red onion. Pour the dressing over the mixture and toss gently to coat evenly.
Let the salad rest for 30 minutes in the refrigerator to allow flavors to meld.
Just before serving, sprinkle the crumbled feta cheese over the top of the salad. If using red pepper flakes, add them now.