In a blender, combine half the watermelon, sugar, lime zest, lime juice and water. Blend fully until there are no chunks remaining.
Strain the fruit mixture through a fine mesh strainer into a 9x13 baking dish. Discard the pulp.
Repeat with the remaining ingredients.
Place the mixture in the freezer on a flat surface and freeze 1.5 hours. Remove from the freezer and stir with a fork, scraping the sides. Return to freezer.
Freeze 1.5 hours more and again, remove, stir with a fork, scraping the sides. Return to the freezer.
Allow to freeze completely. Once fully frozen, use a fork to scrape the ice, forming ice flakes. Scrape the block of ice uniformly to prevent it from breaking apart.
Serve immediately. Store remaining granita in the freezer.