Coco frios are sold on every corner during the high season (fall and winter) in Puerto Rico. Cold, fresh coconut water is the lifeblood of island living. Literally, if you were ever stranded on an island, you could sustain yourself on what's inside a coconut (just ask Les Stroud).
There is a difference between coconut water and coconut milk. Coconut water is clear to translucent and not as sweet—you harvest this from a fresh green coconut that has just fallen, or is ready to fall from the tree. It might taste unusual at first if you have never tried it, but this is actually the best form of natural electrolytes to replenish yourself after a workout or surf session.
Watch the roadsides for the carts and folks with machetes—because this is your hint that coco frios await you!