Using a chef's knife and cutting board, peel the cucumber if desired and dice it into approximately ½-inch pieces, ensuring even sizes for uniform texture.
Slice the seedless watermelon into 1-inch cubes, aiming for uniform pieces without excessive juice loss.
Combine the diced cucumber and watermelon in a large mixing bowl, gently tossing to distribute evenly.
Squeeze the juice of one lemon over the bowl and drizzle with olive oil if desired. Gently toss to coat all pieces, ensuring an even distribution of dressing.
Sprinkle the crumbled feta over the salad and lightly toss again to incorporate. Serve immediately for optimal freshness and texture.
Arrange the salad on plates or a serving platter, optionally garnishing with additional feta or fresh herbs for presentation.