Place the watermelon cubes, lime juice, sugar, and salt in a blender. Blend until smooth, adding a little water if needed to help the ingredients blend.
1⅓ pounds seedless watermelon chunks, no rind, 2 tablespoons freshly squeezed lime juice, 2 tablespoons sugar, pinch sea salt
Strain the watermelon mixture through a fine mesh sieve into a large pitcher or measuring cup. Discard any pulp. Pour in the cold water and stir well. Taste and adjust the sugar or water amounts to your taste. If you strained the mixture into a measuring cup, transfer it to a pitcher for serving.
2⅔ cups cold water
Fill two rocks glasses with ice. Add a slice of lime to the inside of the glass. Pour the watermelon mixture over the ice and serve.
ice cubes, thin lime slices for garnish
Recipe Notes
Make the watermelon agua fresca ahead and refrigerate, covered, until serving. Add ice to the glasses so it doesn't water down the watermelon mixture.