I heard the answer to this once but I’m not sure if it’s actually true.

Apparently it goes that when medieval peasants bathed, they would re-use the water in the tub for the whole family because fetching new water every time would be a huge hassle.

The order in which people bathed was from oldest to youngest so by the time the baby was clean, the water was super dirty and you couldn’t see the bottom of the tub, hense it could be possible to have something in the water that you cant see.