I've seen them at 20-25 ft in the Suwannee. Almost kissed a 12 footer on the right rear ham in about 15' of water in the Waccassa once. I don't know what his reaction was, as I left too quickly to observe it.
I've never been bothered by one while underwater (topside is another story). Most of the ones I've seen have not reacted to my presence at all.