Place watermelon in a zip-top plastic freezer bag. Seal bag; freeze 8 hours.
Process watermelon, rum, and remaining ingredients in a blender or food processor until smooth, stopping to scrape down sides. Serve immediately.
Note: For testing purposes only, we used Cointreau for orange liqueur.
I added more rum and orange liq. I would also use superfine sugar or simple syrup instead of conf sugar, but all in all a good start!