He surmised that if it was too easy for the kidnappers, he, his son, and the kidnap victim might be killed sooner, rather than left to die in the heat.
However, the idea of drinking the water out of the car's radiator would have been fatal. Cars since 1926 had been using ethylene glycol as anti-freeze in the car's cooling system, and it is poisonous.
As for the truck, it was used, and had a misleading label on it, so there was no obvious reason for the kidnappers to suspect it was full of water. But what if the kidnappers had asked, or even checked for themselves? For all his claims, the guy in the suit was not that smart.
Also, you do not die from lack of water quite that fast. Even in the hot sun.
It was kind of sloppy writing, all in service of the twist, which was not that interesting.
It is the acting of James Bell that makes this episode of any interest at all.
Other than the looks, Burt Reynolds does not add much to this. Of course, he was just starting out, and he got better. Much better.