Lots of good advice as usual.
I am using an HC3 in an Ocean Images housing with light package. The camera with lights is slightly negative and I keep it tethered with a spring coil to a D ring around waist level. Fortunately I dive in Hawaii so I don't need all that "wetsuit heater" stuff
As an instructor, I am always obligated to assist divers of whether they are my customers or not. I have needed to ditch the camera on several occasions to "assist" a diver. My system hangs about 1 ft below my fins when I am vertical and it has never been in the way. Left the camera running one time while assisting a panacked diver from 100ft to the surface, video was lousy, but audio was spooky...you could hear the guy screaming the whole time.
I do have to be aware of my distance off the bottom and make sure the camera doesnt drag (clamped between the knees works well in those situations, and if you saw my previous post and have mastered that kicking technique then you can still take care of other issues on the dive).
All of my fine tune bouancy is done with lungs and serious breath holding. As long as you develop the automatic reaction of exhaling ANY time you ascend at all, then holding your breath to hold position is no problem.
btw, I have over 2000 dives with a camera and still have unsteady shots..sometimes s**t just happens.