TIP: For a spicy option: Add a jalapeño pepper to the watermelon when you blend it.
8 cups 1-inch cubes watermelon
6 limes, cut into wedges, plus more for garnish
2 to 4 tablespoons agave nectar
Spread 2 cups of the watermelon cubes on a small plate or sheet tray. Place in the freezer until frozen solid, 1 to 2 hours.
Place the lime wedges in a blender. Fill with water to cover the limes and add the agave. Pulse only 4 times. Pour the mixture through a strainer set over a bowl, making sure not to touch or squeeze any of the limes in the strainer. Pour the liquid into a pitcher and discard the limes.
In the blender, puree the remaining watermelon and strain it, pushing down to get all the juice from the pulp in the strainer. Discard the pulp and add the watermelon juice to the pitcher. Taste and add more agave if needed for sweetness. Refrigerate until chilled.
To serve, fill glasses with the frozen watermelon cubes, top with agua fresca, and garnish with lime.