Now that my son is 17 months and obviously able to sit on his own, he sits directly in our tub. I have been filling the water to about 3 inches. It covers his legs and privates.
My husband is always commenting that we should fill it much deeper. I think he would like for it to be mid-belly. So I guess 6 inches?

Am I being overly cautious? One of us is always there within arms reach. I can't find anything on The Google about toddler water depth specifically.