Coconut water has been found to be a pretty steril and decently effective substitute for rehydration fluid. British and Japanese soldiers during World War II used coconuts in the emergency room. Another report in 2000 was of a stroke patient who was too ill to drink or use a nasal tube. He was successfully rehydrated with Coconut Water.
Although, Coconut water has fewer electrolytes and too much potassium, so it's not ideal and shouldn't be used in most stiuations. Also, we should note that this refers to Coconut water, not Coconut milk, which is grated coconut meat.