If you have a really ripe, gorgeous watermelon, you may not need any sugar at all.
- ½ medium seedless watermelon, rind removed, flesh cut into 2-inch pieces
- ¼ teaspoon ground pequin chile or cayenne pepper
Purée watermelon, lime juice, sugar, and salt in a blender until smooth. Strain through a fine-mesh sieve into a 13x9" baking dish; discard solids.
Freeze watermelon mixture until edges begin to set, 30–35 minutes. Using a fork, scrape to break up frozen portions. Freeze, scraping and breaking up every 20–30 minutes, until mixture resembles fluffy shaved ice, 2–4 hours.
To serve, divide granita among bowls and sprinkle with ground chile.
Do Ahead: Granita can be made 1 week ahead. Cover and keep frozen. Scrape to fluff just before serving.