Start by taking an electric blender, and dropping in the chunks of peeled and chopped watermelon – you might want to buy a seedless watermelon for the purpose of making this cocktail. Once the watermelon is in, add in the golden circle pineapple juice, and put on the lid of the blender, making sure it is absolutely secure.
Turn the blender on, and pulse on medium speed for about 30 seconds, until both fruits have been pureed together, and attain a smooth consistency. Then, remove the lid, and pour in the white rum or Cachaca, triple sec liqueur, and one tablespoon of grenadine.
Secure the lid of the blender once again, and blitz everything at low speed, for around 8 seconds. Once all the liquids have been incorporated into the fruit puree, and the mixture is smooth, with no chunks of fruit, turn the blender off.
Distribute the crushed ice among your eight serving glasses, and then proceed to divide the watermelon mixture equally among all the glasses. Give each glass a final stir, garnish with mint leaves, place them in the refrigerator to cool, and then serve chilled.