Use a chef's knife and cutting board to dice the watermelon into uniform 1-inch cubes.
Peel the cucumber and cut it into similar-sized 1-inch cubes using the same knife and cutting board.
In a large mixing bowl, combine the diced watermelon and cucumber. Gently toss to mix the ingredients evenly.
Sprinkle the crumbled feta cheese over the mixture, then drizzle with olive oil if using. Season with salt and pepper to taste. Gently toss again to distribute the feta and seasonings evenly.
Transfer the salad onto a serving plate or bowl. Garnish with extra feta if desired. Serve immediately for a fresh, crisp texture.