In a blender, add in the watermelon, lime juice and granulated sugar. Blend until pureed. You can strain any of the watermelon pulp, but I left it.
Pour the watermelon juice into a large pitcher.
If you were to add any liquor, do it now.
Pour the cold ginger ale into the pitcher and stir. Serve into four glasses and drink immediately. If you plan on drinking it later, hold off on adding the ginger ale until you plan on serving it. Add ice if preferred.
Notes
Liquor: You can add the desired amount of liquor of your choice just before serving. Storage: Store the watermelon puree without the ginger all in the fridge covered with plastic wrap or in an airtight container. Add the ginger ale just before serving to keep it fizzy.