Place watermelon in a single layer on a plate and freeze for about 1 hour or until solid.
Place watermelon, rum, orange juice, water, and sugar in blender and process until smooth. Garnish with orange slices if desired.
SNAPPY TIPS: If your blender has a hard time running, add a little more liquid such as water. Adjust sugar to your personal taste.SNAPPY SUBSTITUTIONS: You could substitute lime juice and lime slices instead of orange. You could also adjust the rum amount with watermelon and/or orange flavored liqueurs if you have them.