I ask on behalf of Whatsit Jr., who has developed an unfortunate penchant for sticking his face into the bathwater and slurping some up before we can stop him.
I really don’t think anything horrible is going to happen, but I was idly pondering what the worst-case scenario would be. Anyone know?
Worst-case scenario, IMO, would be if you nagged him so much about it that he went “off” baths. 
Seriously, though, I suppose there’s a–IMO–very slight risk of him picking up some Bad Bacteria and getting diarrhea. Also, depending on how soapy the water is, the soap itself could give him tummy trouble.
How old is he? He’s doing it for one of three reasons.
He’s thirsty.
He’s curious.
He’s rebellious.
Get him a special drinking cup just for the bathtub, and make a big deal out of getting him a drink of clean water.
Tell him neutrally, “Ick, we don’t drink bath water”.
Discourage him but without making a big deal out of it.
P.S. If you’re in the habit of letting the family iguana or turtle play/bathe/swim in the tub, the kid can pick up salmonella from it.
Ten minutes ago I was listening to the radio. A woman caller told the story of how she had always wanted to bathe in champagne. For her birthday party all the guests brought round bottles of champagne and filled a tub for her to bathe in. She said that she kept her undies on for modesty. Afterwards the guests refilled their champagne bottles and consumed them. Go figure.
Thanks for the info, DDG. I believe his reasons are mainly #2 and #3, and we have also used #2 and #3 from your second list on him already (they’re basically the same thing). He’s not thirsty, because he has bathtime right after dinnertime, during which he gets the opportunity to drink as much milk and/or water as he wants. I think he just thinks drinking the bathwater is cool.
Anyway, it’s not a major problem for us and therefore we don’t really need advice on what to do (although I do appreciate your thoughts), I was just curious about what horrific things could happen, if any, as a result of drinking your bath.
Well, as long as the iguana isn’t sharing the tub with him, I wouldn’t worry too much.
I have a factoid from my mother in my head that says, “Drinking bathwater will give a kid diarrhea”, but the factoid doesn’t mention why. Factoids are like that.
My son is the same, probably because he is a little thirsty in the evenings (I’m trying to cut down hi evening fluid intake to keep him dry through the night).
The bacteria at the lower end of the digestive tract can wreak havoc if they get into the top end; in theory, he could end up ingesting some of his own fecal flora this way, but in practice it is probably pretty unlikely to be problematic due to the low concentration and the antiseptic action of the soap.
At a wild guess, I’d say it is inadvisable, but probably safer than using the toilet, not washing your hands, then eating finger food.
My niece used to be the same, shed drink bathwater (even to the point of sucking the flannel/sponge) and even get at the water in the washing machine. She stopped doing it of her own volition and was never noticeably harmed by it.