Cook corn in hot boiling salted water for 5 minutes. Remove and allow to cool. Chop watermelon, crumble feta and chop mint while corn cools. Cut corn from cob. Discard cobs. Toss corn, watermelon, and crumbled feta in a large salad bowl. Whisk together rice wine vinegar, olive oil, chopped mint in a small bowl. Salt and pepper to taste. Toss salad with dressing until thoroughly dressed. Cover and chill for about one hour until ready to eat.