After I asked that question and read a lot of replies, I started wondering about this.
I stay right beside the tub the entire time my son is in there. After I wash him, if he wants to play for an hour, I stay; he is 3.
That being said, I agree that your child could fall, hit their head and drown in the few seconds you are gone, but couldn't they do that while you are right there? I have non slip bath mat and I make my son sit everytime he stands, but it only takes a second. That's super scary to me!