Start by gathering your ingredients: cubed frozen pineapple, coconut water, and rum. If desired, set aside two pineapple cubes for garnish.

In a blender, combine the coconut water and rum, blending on medium speed for about 15 seconds.

Now, add the cubed frozen pineapple to the blender and blend on high for 30 to 45 seconds until smooth.

Taste your slush and adjust by adding more rum if desired, then blend for an additional 5 seconds.

Pour the slushy mixture into highball glasses, serving immediately, and add garnish if using.