Place sliced red onion in a small bowl of cold water for 5 minutes. Drain and pat thoroughly dry with paper towels.
Whisk lime juice, olive oil, honey, flaky salt, and black pepper together in a small bowl or jar until the honey fully dissolves.
Cube the watermelon into 1-inch pieces and place in a large mixing bowl. Add the cucumber slices and drained red onion.
Pour the dressing evenly over the bowl and toss gently two or three times to coat. ⚠️ Food Safety: Assembled salad with cut melon should not sit at room temperature for more than 2 hours.
Scatter the crumbled feta across the top, then add the torn mint leaves. Fold once or twice from the bottom up. Serve immediately.