Cut the watermelon into cubes and remove any seeds if necessary.
Place the diced watermelon in a blender or food processor and pulse a few times to break it down into small chunks (not fully smooth).
Pour the watermelon puree into a shallow baking dish.
Stir in the granulated sugar and fresh lemon juice after pouring the watermelon puree into the shallow baking dish, before placing it in the freezer.
Place the dish in the freezer at a standard freezing temperature (0°F / -18°C) and let it freeze for 30 minutes.
After 30 minutes, scrape the mixture with a fork to break it into crystals.
Continue scraping the granita every 30 minutes until it reaches a fluffy, granular texture, which should take around 2-3 hours.
Serve in bowls or glasses and enjoy!