Make fresh watermelon juice by tossing cubed watermelon in a blender for 15-30 seconds. If you prefer, you can strain the watermelon juice to remove any stray seeds or pulp.
If you don't have an orange to juice, you can use OJ from a carton or substitute with orange liqueur instead.
If you prefer your cocktails boozier, add an extra ounce of tequila.
TO MAKE FROZEN: add 3 or 4 ice cubes to the blender with the remaining ingredients, and pulse until smooth. Pour into glass, and serve immediately.