Scoop watermelon flesh out of watermelon (a metal spoon works great) to create the "bowl" and add to blender. Leave about an inch of watermelon at the bottom for stability.
Blend watermelon until smooth. Add ice to blender if watermelon has not been refrigerated.
Add Malibu Rum and lime juice to blender. Blend until mixed.
Pour mixture into watermelon bowl.
Garnish with mint leaves, and add an umbrella and a milkshake straw.