Whisk vinegar, honey, salt and pepper in a small bowl; gradually whisk in oil until completely incorporated.
Combine watermelon, cucumber, feta and onion in a large bowl. Gently stir in ½ cup of the vinaigrette. Refrigerate for at least 20 minutes or up to 2 hours.
Just before serving, gently stir in mint; drizzle with the remaining vinaigrette.
To make ahead: Prepare through Step 2 and refrigerate for up to 2 hours.
Nutrition information
Serving size: 1½ cups
Carbohydrate Servings: 1
Exchanges: 2 fat, 1 fruit, 1 medium-fat protein, ½ vegetable