Place the cut watermelon in the blender with the sugar, water and lime juice. Blend at high speed until smooth. Optional: Pour the drink through a fine mesh strainer into a large pitcher. (I strain other types of melons but haven't found it necessary with watermelon.) Note: If you're making a double batch and it won't all fit in the blender, but what you can do instead is add just part of the water to the blender with the melon and sugar, pour the mixture into your serving pitcher, and then stir in the remaining water. Chill in the fridge for at least an hour. To serve, fill glasses with ice cubes, pour in the watermelon agua fresca, and garnish with a mint sprig and/or a slice of lime.
Note: The ratio of fruit to water and sugar can be changed to suit your personal preference.