Add the watermelon cubes to a blender and blend until smooth.

Strain the watermelon puree through a fine mesh sieve to remove any pulp.

In a cocktail shaker, combine 4 oz of the watermelon juice, white rum, lime juice, and simple syrup.

Fill the shaker with ice and shake well until the mixture is chilled.

Strain the mixture into a chilled cocktail glass.

Garnish with a small slice of watermelon or a lime wedge if desired.

Serve immediately and enjoy.