Cut the watermelon and feta into 1-1/12″ cubes, place in separate bowls. Finely chop the mint. Toss the watermelon in the balsamic, then the chopped mint (you can reserve some to sprinkle over the finished skewers). Thread three total pieces of watermelon and feta onto the skewers in an alternating pattern. Arrange on a platter, sprinkle with smoked salt. Serve well chilled.