Remove rind and chop watermelon into 1" pieces. Finely chop mint.
Combine watermelon pieces, feta, and mint in a large bowl.
In a small mixing bowl, whisk together lime juice, balsamic vinegar, olive oil, and salt until thoroughly combined. Pour over watermelon mixture and toss well.
Let sit for about 30 minutes before serving (but no more than an hour or so to ensure it stays fresh).