In the container of a blender, add chopped watermelon, sugar, lime juice, salt, and cayenne, and blend until smooth. Strain mixture through a fine-mesh sieve into a 13×9-inch freezer-safe baking dish, discarding solids. Stir in zest. Freeze until edges begin to set, 30 to 35 minutes.
Using a fork, scrape mixture to break up frozen pieces. Freeze until mixture resembles fluffy shaved ice, about 4 hours, scraping to break up frozen pieces every 30 minutes. Serve in sugar-rimmed glasses with watermelon wedges. Garnish with zest, if desired.
Recipe by Louisiana Cookin at https://www.louisianacookin.com/watermelon-lime-granita/