Cut a seedless watermelon into small cubes until you have 10 cups of watermelon cubes. Spread the cubes in a single layer on a rimmed baking sheet lined with parchment paper and freeze overnight or at least 24 hours until solid. Transfer the frozen cubes to a freezer bag for storage.

Add 10 cups frozen watermelon cubes, 2 tablespoon maple syrup, juice of 1 large lime, ¼ cup fresh mint leaves (optional), and 1½ cups filtered water to a high-speed blender. Pulse a few times to start breaking up the frozen watermelon.

Blend until the mixture forms a thick slushie texture. If needed, add a little extra water and blend again. Taste and add more maple syrup if your watermelon needs more sweetness.

Pour into 4 glasses and serve immediately while the slushie is thick and icy.