There was a post about this long ago(I will see if I can find it) and I had thought that they did not hydrate that way and I was corrected by Dr. O who is one of the vets who participates on this forum. It is not the standard way to hydrate a chameleon, but apparently they can benefit from it. I think if your chameleon was ill and dehyrated it would be more of a last resort and not something you want to do on a regular basis. Misting and using a dripper are the normal ways to hydrate of course. If I can find the thread I will post it..
ok so here it is. It was actually a thread about constipation but talks about baths and hydration also.
As quoted by Dr. O:
-if your cham goes berserk in water, then i absolutely agree---too much stress!! however, ime some chams will either slowly walk around in belly-high lukewarm water, while most will stand up on 2 legs and do their slow-motion scrabble with their front feet trying to climb out (but not show any real stress colors).
the reason i prefer to keep mine "dunked" is twofold. most obvious is that it's warming their belly nicely and getting things moving. but the main reason is that the cloaca can absorb a tremendous amount of water, virtually as hydrating as oral. so not only are they absorbing extra water which helps everything, but it's also coming at the fecal from the other direction, helping to soften it even more. mine don't usually poop in the water (10-15 minutes total), but within the hour after it's almost always successful.