In batches, combine 12 cups water, the lemon juice, sugar, jalapeno, watermelon and mint, if using, in a blender. Blend to a puree.
Allow the puree to steep for 10 minutes before straining it through a fine mesh sieve. Chill well.
Serve chilled, with ice and a lemon slice floating at the top.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.