Many athletes use coconut water in their training and also some tropical marathons like the Reggae Marathon serve opened coconuts after the finish line.
This is probably due to the fact that coconut water contains some carbs, protein and fat as well as minerals (potassium, sodium, phosphorus, magnesium, calcium, zinc, iron, copper, manganese, selenium) and vitamins (C, B).
Coconut water is a natural isotonic beverage, as it has the same salt concentration as the cells of the human body and blood. Therefore it may be consumed to replace fluids lost during physical activity.
Recently I've also started using coconut water as a recovery drink after hard exercise. It seems to work pretty well, although obviously an athlete needs to eat something with more calories (120 cal/500 ml) in addition to that.
It's possible to blend a fortified home-made natural sports drink with some dates or any dried fruit (for sugar) and celery (for sodium). This recipe also works with any normal water.
Green coconuts are rarely available at local markets, except possibly some ethnic stores which may occasionally have them. People also generally find coconuts inconvenient to open, unless they own a machete or some old big knife. It's not worth spoiling your good kitchen knives with coconuts. Also the old nail and hammer trick works quite well.
I've not been too interested in investigating commercially packaged coconut waters, because manufacturers always seem to be forced to use preservatives and pasteurisation. That's totally unacceptable for purists like me.
The other day I noticed Dr. Antonio Martins Coco Juice while shopping around. The half-liter carton claimed 'pure organic coconut water, not from a concentrate, no preservatives'. So I bought and drank one right away and thought the taste is not bad, although definitely not the same as real fresh coconut water either.
From their website I learned that Dr. Martins is using a patented cold-process technology to pass the coconut water straight through a filter into the carton. Thanks to this invention heat treatment can be avoided. That's certainly a step forward.