Place all ingredients in a blender and blend until smooth.
To make prep for these slushes easy I like to buy an extra watermelon when their on sale (or when they are $4.99 at Costco) and dice up the second 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, or if it's too tart add a little sugar at a time until desired sweetness is achieved.