I know 2 children who drowned in the bath... one 3ish and the other 6. The 6 year old hit their head and was convulsing in the tub, so sounded like they were happily playing. While I know that can happen at any age, it made a profound impression on me. At 7 nearly 8, I let kiddo bathe by himself about 1/2 the time... but I still talk to him periodically (every few minutes... a person can go without o2 to the brain for a few minutes without damage, so that's my guide). The other half I sit and we talk about his day or he tells me stories or I just read a book. I may seem paranoid, but I feel pretty relaxed about the whole thing. Then again, I grew up with public baths the size of swimming pools and everyone naked... so the whole "privacy" thing just isn't a family standard in our house. It's not like he's doing anything in the tub/shower that I shouldn't see after all. Older, sure... but he's a KID. When he's a teen, he'll get privacy. Until then, I'll keep as much of an eye on him in the tub as I do when he's in a pool.
BTW... until Kiddo outgrew it (at around age 3.5) I used to fill up one of the BIG plastic toy buckets (the kind with rope handles) to about bellybutton level with water, and then several inches of bubbles and drag it/him around the house with me while I cleaned or cooked. Started it a few months after he was sitting up on his own comfortably.