Cut the watermelon flesh into 6 (1 1/2-inch) cubes.
Use the small end of a melon-baller to remove a scoop of flesh from 1 side of the watermelon cubes, creating a small cavity for the juice.
To serve, place a watermelon cube, cavity-side up, on each of 6 small plates and squeeze a little lime juice over them. Fill each cavity with balsamic vinegar.
Recipe courtesy of Rick Tramonto, Amuse-Bouche: Little Bites That Delight Before the Meal Begins, Random House, October 2002