If a cham opens his mouth wide enough, you can see directly into his lungs.
So it would be easy to accidently get something in the lungs.
Weather a little water would bother them or not, I don't know, but I have seen videos of chams 'swimming' so I know they must be breathing in some water in the wild w/o harm.
I would not worry about small amounts, and they can cough stuff back up if they have to.