Place watermelon in a blender or food processor, and blend until pureed.
Strain through a fine mesh strainer into a bowl or liquid measuring cup.
In a small bowl, juice the limes.
Add the mint to the lime juice and muddle the mint, either with a mortar and pestle or just mash it with a fork.
Place 3-4 tablespoons of strained watermelon juice into each glass with ice.
Add 1 tablespoon of lime/mint juice to each glass.
If making it a cocktail, add vodka to each glass.
Top off with ½ can of sparkling water.
Serve and enjoy!