Place all ingredients in a blender and blend until smooth.
Video
Notes
Watermelon Slushy Pro Tips:
To make prep for these slushes easy, I like to buy a second watermelon when they're on sale (or when they are $4.99 at Costco) and dice up the extra one. I measure out the pieces into 5-cup portions and freeze them in Ziploc bags. This way, whenever the hankering for a slush comes on, it only takes 2 minutes to make.
If the slush is too sweet for you, add a few more cubes of watermelon. If it's too tart, add a little sugar at a time until desired sweetness is achieved.