In a blender, add watermelon, lemon juice, sugar, and 2 cups of ice. Blend until roughly combined (ice chunks are ok!). Add the remaining cup of ice and blend until smooth.
Pour into a shallow pan (9x13 works great) and put in the freezer. After 30 minutes, take a fork and rake the mixture. Repeat after 1 hour, and again 30 minutes to an hour later. When ready, the mixture will be frozen like a slushy, but not solid.
Scoop into a cup and top with your desired amount of sparking water for a refreshing mocktail! You can also enjoy the granita on its own like a sorbet.
