Slice 3/4 of a seedless watermelon into cubes, reserving one quarter for juice, and place in a bag and freeze for several hours or preferably overnight.
In a strainer over a bowl, press the remaining chunks of fresh watermelon with a muddler or flat surface until most of the juice is extracted.
Slice limes into quarters and squeeze the juice of one fruit into each serving glass, throwing the limes into the glass along with it. (Only use half of a lime instead of 2 medium limes if you prefer a drink that's not as tart.)
Add 1 tbsp sugar and one shot (2 Tbsp // 30 ml) of white rum to each serving glass and muddle until well mixed.
Fill to the brim with watermelon ice cubes, add a healthy splash of watermelon juice (about 1/4 cup), and top off with club soda.
Stir, garnish with lime wedge, and serve with a straw.
*Only add half of a lime if you prefer a less tart drink. *If your watermelon isn't frozen, just add in a few actual ice cubes to chill the drink. *You'll have enough watermelon ice cubes for at least four drinks. *Omit the sugar if you prefer a more tart cocktail. *I highly recommend eating the boozy watermelon cubes as they melt - so tasty! *Questions about substitutions, equipment, or troubleshooting? Check out our Recipe FAQ page. *Nutrition information is a rough estimate.