Put cubed watermelon in blender and process until pureed. You should get about ¾ cup of watermelon juice.
Place a fine mesh strainer over a large bowl and pour watermelon juice in to strain out any pulp, seeds, or foam.
In an ice-filled cocktail shaker, add watermelon juice, coconut water (or plain water), simple syrup (or agave or honey), and lime juice. Shake well.
Pour entire contents of shaker (ice included) into a glass. Add a slice of watermelon and a lime wedge for garnish and enjoy!