I would think 5-6 foot deep are pretty common. I've stepped in a few of those by mistake. Then, like BRF said, you have the 10-12 foot deep holes that just look dark or even blue and you can't make out bottom real well.
I always think there must be some real pigs in those big holes, the one headed up East prong to Elkmont comes to mind first. I've fished it a few times but if the weather is decent there's almost always people swimming in it. I always imagine a 5 lb. Brown laying down there though, just slurping whatever comes down the river bend.