Slice watermelon and place in the freezer for 4 hours to freeze.
Remove frozen watermelon from the freezer and place in a blender. Puree (you can add 2-3 tablespoons of water to help the blender along if you need to but you want it to remain a pretty firm slush.)
Pour equal portions of watermelon slush into 4 glasses.
Pour 3 ounces each of Prosecco or other sparkling white wine over the watermelon slush in each glass. Stir gently to mix.