Patillazo is what Barranquilla drinks in the heat: watermelon blended with water and crushed ice, then sharpened with lime juice. Patilla is what the Caribbean coast calls watermelon. It is sold from street carts and ice cream parlours, and it is everywhere during Carnival. Makes one litre.
If you want it sweeter, add sugar to taste and adjust from there. Watermelon varies a lot in sweetness, so taste before you add anything. It separates as it stands, with the pulp settling and the water rising, which is normal: stir it or shake the jug before pouring. Make it close to when you are going to drink it, since blended watermelon loses its fresh taste within a couple of hours even refrigerated.