Whisk the dressing: In a large salad bowl, whisk together the lime juice, olive oil, salt, and pepper until smooth.
Add the salad ingredients: To the same bowl, add the watermelon, cucumber, arugula, onion, and mint.
Toss to combine: Gently toss everything in the dressing until well coated.
Finish with feta: Sprinkle the feta on top and give a light toss just before serving. Taste and adjust seasoning if needed.