Add the watermelon, cold water, and ½ of the sugar to a large blender. Blend until completely smooth.
Taste and add the remaining sugar if needed.
Serve immediately over ice or chill in the refrigerator for up to 3 days. Garnish with lime wedges and mint sprigs if desired.
Notes
If you prefer a smooth agua fresca without any froth or pulp from the watermelon, I recommend straining it through a fine mesh strainer before serving.
Can’t finish your agua fresca in 3 days? Freeze the rest in ice cube trays for up to 3 months.