Remove the skin and seeds from the cantaloupe and dice the flesh. Put in a blender and blend until smooth.
Strain the puree through a very fine strainer or cheesecloth so that you get a nice clear juice. You should get around 1 ½ cups/360ml. Mix this with the water and lime juice. You can add a little sugar if you like but I don't think it's really needed if your cantaloupe is ripe.
Serve over ice with a little mint as garnish.
Some agua frescas have a higher proportion of water but to me, this works well - if you want to dilute more, you can. Also, if your melon is not that ripe or sweet, you may want to sweeten slightly.