Place the diced watermelon in a food processor or blender and pulse until broken down into a smooth, even puree. Transfer to a mixing bowl.
Add the sugar and lemon juice to the watermelon puree. Mix thoroughly until the sugar dissolves completely and the mixture is well combined. The liquid should have a slightly frosted appearance.
Pour the mixture into a shallow freezer-safe dish, spreading it into an even layer. Place it in the freezer.
Freeze the mixture for 1 hour, then remove it from the freezer. Use a fork to scrape the surface, breaking up any ice crystals and creating a flaky, granular texture. Return to the freezer.
Repeat the scraping process every 30 minutes for about 2-3 hours, until the entire mixture is frozen into a light, crystalline texture resembling shaved ice.
Serve the watermelon granita in bowls or glasses, using a spoon to scoop up the icy crystals. Garnish with fresh mint or a slice of lemon if desired.