I think you are referring to coconut water and not coconut milk. Coconut milk is normally used for cooking while agua de coco is drunk as a beverage.
Coconut water - Wikipedia, the free encyclopedia
It would not have nearly the sugar nor salt content that is mentioned in the OP.
Coconut Water would be a great and much cheaper substitute for the Gatorade part of the original post. Coco de Agua is many times better than Gatorade and similar drinks for replenishing electrolytes and potasium. And it's all natural. I've even read that Coconut water could even be substituted for a drip IV in a pinch.
I found it!
(Coconut water is also used as an intravenous hydration fluid in some developing countries where medical saline is unavailable.[1]) From Wikipedia.
Coconut water vs. Gatorade | Flex | Find Articles at BNET
Coconuts May Be Nature's Gatorade - ABC News