If preparing the mint lime sugar, zest the limes and add to a small bowl. Add the mint leaves, sugar, and dash or salt and mix well. Cover and store at room temperature until ready to use.
For the granita: place watermelon and honey into the blender. Add the juice of both limes and blend until well combined. Give the mixture a quick taste test to see if additional sweetener needs to be added.
Using a fine mesh strainer, strain the watermelon mixture over a 9x9 freezer safe container.
Freeze the watermelon juice for two or three hours, then begin the process of lightly scraping the top frozen layer with a fork. Return pan to freezer with the shaved ice on top; remove a couple of hours later and continue scraping. Repeat the occasional, two to three times, scraping process until the entire mixture is shaved.
Scoop out granita into serving dish and sprinkle with a teaspoon of the mint lime sugar mixture.
Notes
Granita will stay in the beautiful icy consistency in the freezer in an airtight container for up to 2 months.