Place watermelon cubes in a single layer in a shallow pan; freeze until firm.
Remove watermelon from freezer; let stand 5 minutes. Drop watermelon through food processor chute with processor running; process until smooth. Add chunks of frozen raspberries alternately with sparkling water; processing until mixture is smooth.
Tip: This is a great recipe to make when you have half of a watermelon left over. Or you can substitute containers of cubed watermelon from the produce section of the supermarket.