Place rum in a martini shaker with a splash of water and lots of small ice cubes.
Shake shake shake and drain into a chilled glass with a cube or two of watermelon.
Garnish with a wedge of lime if desired
Notes
SlushieBlend watermelon with rum and a few ice cubes in a blender to desired consistencynote: add lime juice, or lemon lime soda if desired for either cocktail