Well, that isn't 100% accurate. It doesn't just force water into the vagina, it forces it also into the uterus a far far more dangerous place for chlorinated water to be. In the vagina, it would simply drain out...in the uterus, not so easily. Anyhow, this was explained to me by my gf's gynecologist......so I trust her over anything found on the net......
Like I said, I'm not sure if toxic shock syndrome is the proper term, but it can be pretty dangerous to have chlorinated water sitting in the uterus for any length of time. I also think she said something about the water being forced into the fallopian tubes as well, but don't quote me on that.....
In addition, my doctor also told me about sex in water can force water into the urethra (male and female) leading to severe bladder infections.
So, it isn't a good idea in any sense.....