Also known as agua de tuna, this is a refreshing drink on ice. If it's cocktail hour, add tequila or mezcal. Once made, it will keep a few weeks in the fridge. Left on the counter, it will ferment in a day or two.
Remove the spines from the fruit, as noted in the text above. Put the fruit in a pot and just barely cover with water. Bring to a boil. Turn off the heat, and when it cools enough to work with, mash everything with a potato masher.
Run the mash through a food mill, or of you don't have one, push it through a colander with a rubber spatula. You need to get rid of the many seeds. Once that's done, strain the mash through a fine sieve, pushing it through with a rubber spatula. If you want a very clear juice, run that one more time through a sieve with a paper towel set inside it, and do not push it through -- just let it drop through over the course of an hour or two.
Add sugar, plus citric acid or lime juice to taste. Enjoy over ice.