Place all the ingredients in a shaker and shake until very well chilled, about 30 seconds. Strain into a chilled martini glass.
*To make frozen watermelon cubes, using a small melon ball scoop, remove flesh from watermelon. Place balls in an empty cube tray or on a foil-lined plate and cover with plastic wrap to seal in flavor and avoid watermelon absorbing food odors. Place in freezer for 1 1/2 to 2 hours. Use as needed.