Put the cantaloupe, mint, agave, lime juice and two cups water in a blender and blend on high until it’s smooth. Strain the mixture into a pitcher: either line a colander with cheesecloth or use a fine-mesh strainer. Discard the solids. Stir in three more cups water, and taste. You might want to stir in a little more agave if you want the agua fresca a little sweeter. Chill and serve cold.