If you don't know just how little water it takes to drown in, then the risk isn't obvious. A newborn that can't easily move and roll over can drown face down in an inch of water if they manage to get into that position, which even gentle sliding around in a baby tub can facilitate.
Have some advice from this experienced dad with 12 kids: the baby tub is a poor way to clean a baby. The water instantly gets disgusting from poo or whatever it is you are trying to remove, and then that gets all over the baby. Plus the baby tub costs money and takes up space. It's more clutter to trip over in the dark.
Babies can be washed in sinks, with the drain open and the water running. (don't let them kick the water temperature control though) Babies can be washed in a normal shower, held in the arms of a parent.
We did the sink when they were really little, then switched to the big tubs that sit on top of the sink because the kids loved it, the warm water, splashing, or just vegging out as we poured warm water through their hair. But yeah, plenty of times it was a double bath: The 1st one, and then the second one where we got them clean after draining pee water.
