Peel and slice your watermelon into small thin slices. Place on a wax paper lined baking sheet. Place the sheet in the freezer to freeze your watermelon. Remember, the thinner the slices, the faster it'll freeze.
While your watermelon is freezing, make your simple syrup. Add sugar, water, and the zest from 1 lime into a sauce pan.
Heat until boiling, reduce heat and simmer, stirring until all sugar is dissolved. Remove from heat.
Allow the syrup to cool completely and then place in the refrigerator to cool even further.
Once the watermelon is frozen completely and the syrup cooled, add both to a blender along with the Malibu rum and juice from all limes.
Blend until smooth. If you'd like it to be even icier, add in some cubes of ice and blend.
Enjoy with an extra squeeze of lime into your glass if you prefer.
Video
Notes
The thinner you slice the watermelon, the faster it will freeze
The key to a good slush is to make sure all of your ingredients are very cold, with the watermelon being completely frozen.
You can opt to blend your watermelon and then freeze in ice cubes if you prefer.
Do not freeze the syrup, your final result won't taste the same.