I wonder how they would ever have known to go thru that choke point to begin with. If it had even a small amount of water in it, it would appear as a puddle to them, leading to an abyss if anything. I suppose the alternative towards where they came from looked as frightening as where they were about to go.
Plus, the coach is relatively inexperienced: he's only 25. The (front end of the) cave system is a very popular tourist and kids-adventure spot, and he might have thought it was safer than it turned out to be.
I suppose the coach's thinking was, "It's filling up with water behind us (from the sudden monsoon rain)--- the only way we can go is forward." Even if they're squirming through a passage as narrow as a coffin.
My God. Please Lord, keep the trapped people safe. Them, and their rescuers.