Add all ingredients into a food processor or blender. If adding mint, blend until mint is finely chopped and watermelon is pureed. Pour into metal baking dish and cover tightly with foil. Freeze for 1-2- hours, then use a fork to "rake" the mixture into a soft, fluffy texture.
Return to the freezer for another 1-2 hours and rake again. When you're ready to serve, simply remove from freezer again, rake, and fill glasses. I like to rim my glass with a little salt or sugar and garnish with a sprig of mint. **Leftovers can be stored in the freezer for about a week after making.
Make it a cocktail: If you're looking to enjoy a real mojito, simply add 1/2 cup of white rum to your watermelon mixture before freezing. Serve with another ounce of white rum on top. The consistency will be more of a slushy, but just as enjoyable and refreshing!