How to Make It
In an 8-inch-square glass baking dish, mix the balsamic vinegar and water. Freeze, stirring every 20 minutes with a fork, until the crystals are no longer slushy, about 1 1/2 hours.
On a large platter, arrange the sliced tomatoes and mozzarella in a slightly overlapping circle; season with salt and pepper. Scatter the cherry tomato halves, watermelon cubes and basil on top and drizzle with the olive oil. Scrape the granita over the salad with a fork, and serve.