Line one or two baking sheets that will fit in your freezer with wax or parchment paper and set them aside.
Cut the skin and rind from the watermelon, and cut it into one to two-inch cubes. Lay the cubes on the prepared baking sheets.
Carefully place the baking sheets in the freezer, and let the watermelon cubes freeze for about three hours or until they are firm when pierced with a knife. They don't have to be frozen solid. This will make them too hard to blend.
Transfer the watermelon cubes to the bowl of a food processor and pulse about 10 times until they are mostly broken down.
Add the sugar, lime zest, and lime juice and pulse another five or six times to combine everything. The mixture should be thick and slushy. Serve immediately, garnished with minced mint leaves.
Store any leftovers in a covered container in the freezer. Before serving, let sit out for 20 minutes or microwave for 20-30 seconds so ice is easier to break apart and serve.
Notes
Add more sugar for a creamier, less icy texture.I don't recommend honey for this recipe because it will clump and harden when it comes in contact with the frozen watermelon. Sugar or monkfruit dissolves easily and works best.Freeze leftovers. Let thaw for about 20 minutes for easier serving.