Place four cups of watermelon cubes into a blender with a quarter-cup of water and a quarter-cup of simple syrup. Blend until smooth.
Pour the watermelon water through a fine mesh strainer into a pitcher to remove the pulp and seeds.
Repeat with the remaining watermelon, water, and simple syrup. Pour the lime juice into the pitcher and stir to combine. Taste for sweetness and adjust as needed.
Pour into ice filled glasses, garnish with mint leaves, fresh watermelon wedge, or lime wheels if desired.
Notes
Watermelon agua fresca is best enjoyed immediately after it’s made, but it can be stored in the refrigerator for up to 3 days. The texture and flavor of the agua fresca may change over time and the watermelon pulp may settle at the bottom of the container. If this happens, give it a good stir before serving.
Add 2 ounces of tequila, vodka, or rum, to each glass and stir to combine for a spiked drink.
I like to pour any leftover drinks into a zipper-topped freezer bag, squeeze out all of the air, and store it laying flat in the freezer for up to 3 months. Defrost overnight in the refrigerator.
Some watermelons are sweeter than others, adjust the amount of sweetener used to your liking.
If you do not have simple syrup, substitute a half-cup of granulated sugar + an additional half-cup of water.