When mine are in the bath, I sit on the computer (in the study) but I still have my eye on them - they're in my direct line of sight.

Mine are 8 and 6.

I will let them shower alone, but not bath - well they are alone in the room, but I can still see them.

They're more into showers now anyway, which is much better if they want privacy.

I personally know of a nearly 2 yo who was left in the bath and drowned in less than 30 seconds. It was awful.