Cut the flesh of the watermelon into 1/2 in size cubes.
Make sugar syrup:
Melt the sugar in a small pot with the water, then let cool. Puree watermelon with cooled syrup and vodka in a blender in batches, or with a stick blender. Pour into 9 x 13 glass pan, and freeze. Stir every 45 minutes until completely frozen.
Scrape with fork into serving dish.
Adapted from Initiales JB