What about the possibility that he was hustling and couldn't cool down due to the humidity. Sweat doesn't help if you don't have evaporative cooling. He dehydrates due to sweat, but it doesn't do enough good and then he is out of water; maybe early on.

Then, if he doesn't find additional water and has no cell coverage, it's a pretty bleak situation.

As noted above, being close to water doesn't help you if you don't know it is there.

I think we all forget that the human body is a great enigma. Sometimes the body can withstand tremendous amounts of abuse. Sometimes it takes almost nothing at all to kill us.

-john