Working in batches, puree watermelon chunks in a food processor or blender until smooth and no chucks are left.
Put the puree into a separate bowl & continue until all watermelon chucks are blended.
In a small sauce pot, heat the water over medium heat. Once small bubbles appear on the bottom of the pot, add the sugar - no need to bring to a boil, just want it hot enough to melt the sugar. You're making simple syrup!
Once sugar is dissolved, add simple syrup to the watermelon puree along with the lime juice and stir to combine.
Here's where patience and TLC come in. After an hour in the freezer, pull the mixture out. Using a fork, scrape along the sides (those will likely be starting to freeze) and the middle (which will likely won't be frozen yet :)). Cover, and place back in the freezer.
Repeat this process about every 30 minutes to 1 hour. The more you scrape the ice mixture, the finer the ice will be - thus, the less you scrap, you'll have bigger flakes of granita. After at least 4 hours, and multiple turns of the fork, the granita will be ready!
Serve into bowls, top with fresh mint and enjoy!
Notes
If using a seeded watermelon, strain puree through a sieve to remove those pesky black seeds. Also, the white, softer seeds might show up in the seedless version but no need to worry about straining.
Recipe by Pastured Kitchen at https://www.pasturedkitchen.com/recipes/watermelon-granita/