In a blender, add the diced watermelon, cold water, lime juice, and agave syrup.
Blend the mixture on high speed until smooth and well combined.
Taste the agua fresca and adjust the sweetness or tartness by adding more agave or lime juice, if desired.
Strain the agua fresca to remove any pulp or seeds.
Use a spoon and squeeze the bag to extract as much liquid as possible. Discard the remaining pulp.
Refrigerate the strained agua fresca for at least 1 hour to chill.
When ready to serve, fill glasses with ice cubes and pour the chilled watermelon agua fresca over the ice.
Garnish with fresh mint leaves, if desired, and serve immediately.