I wondered if this was based in any kind of fact. Here is some information I discovered, and links to the information:
Monday, Nov 24, 2003 states:
"It (coconut juice) is also considered a close substitute for blood plasma since it is sterile, cool, easily absorbed by the body and does not destroy red blood cells. To quote Morton Satin, Chief of Food and Agricultural Organisation's Agricultural Industries and Post Harvest Management Service: "It is a natural isotonic beverage with the same level of electrolytic balance as we have in our blood. It is the fluid of life, so to speak."
It is important to note that we are talking about the liquid inside the coconut—the juice--not coconut milk which is thick and milky white, made from the meat of the coconut.
Also, the juice can work as hydration when needed in emergency situations.
Go to this web site to read a little more the Heroic Coconut. There's also some great info in case you ever knock out a tooth!
The Hawaiian Island Detective Club
Book One—Pineapples in Peril
Book Two—Menehunes Missing