Combine all ingredients in a blender and pulse until completely liquified. If you used a watermelon with seeds, strain before placing in a freezer-safe container. Taste for sweetness, it should be slightly sweeter than you would drink it at room temperature, as frozen drinks taste less potent when cold. Once it tastes just how you want it, place it in the freezer for 3-5 hours. Three hours will give you a slushy like consistency, and five will be more like a typical shaved ice. If freezing for 5 hours or more, place on the counter top to warm for 10 minutes before serving.