I think you are obliged to provide working bathing facilities whenever possible, so unless there is a true emergency turning off the water would be rude. If you were afraid that they would stop up the commode, would you suggest a chamber pot will be enough or they should go behind a shrub?
If you don't want to host, then change your mind and refuse.
I'm going with MOM1SON, if your home is in such a delicate state than some water on the floor from a shower will cause damage below even if you check on it and mop it up in a timely fashion, you shouldn't host. Water on a bathroom floor is just not that unusual an occurrence, people get out of the shower wet, shower curtains come away from the wall, commodes get stopped up, sinks overflow and then you mop it up and move on.