I Agree with kerrybelle.

After a particularly hairy experience with heat exhaustion/dehydration I learned the hard way that water/H20 was not enough. You have to have something - either food or replacement fluids, in your system to retain the water otherwise it just sweats out and is not used by the body. I tend to use GU or Balance bars and Sports Beans, and sometimes PB sandwich or fruit to help with water retention. Obviously salt (from crackers or pretzels)is also important.

For your friend who's picky about what he puts in his body - it's OK if he's just sitting on the couch but if exercising heavily, just got to take in salts and sugars to maintain.