Line a sheet pan with parchment paper. Spread cubed watermelon into a single layer. Freezer for 2 hours, or until solid.
In a blender, add lime juice, mint, frozen watermelon, sweetener (optional - add to taste. In-season watermelon may be sweet enough on its own), and half of your water. Blend until smooth, adding additional water as needed to achieve your desired slushy consistency.
¼ of a large seedless watermelon = about 32 ounces of cubed watermelon
Maple syrup can be substituted with any sweetener you have on hand (granulated sugar, honey, brown sugar) and can be omitted if your watermelon is in-season and very sweet. Out of season watermelon will need the added sweetener.
Your homemade slushie can be frozen for up to 2 hours, but will freeze solid over time.