I would say that it depends on the type of boat you are diving from, the type of camera and the conditions of the sea.
Different boats, different entries, most common being giant stride and back-roll.
If I have a small compact camera with no strobe, and the boat has a nice platform for a giant stride then sometimes I simply do a giant stride into the water with my camera in my left hand with my arm stretched above my head, that way the camera doesn't get a hard hit from the water (actually it almost stays dry if you do a proper giant stride). If you do this with a weight-belt full of lead, you might loose your belt. I dive with very little weight so this method works for me.
If you can only do a backroll then it depends on your dive-gear as well, I would not hold a camera to my chest while doing a backroll if i have lots of metal buckles etc in the front that can scratch the housing.
I have a large pocket on my wetsuit thigh, big enough for a small compact, sometimes i just put it in there and leave it there until I need it/if I need it.
Conditions of the sea, if you've got a nice current running then you might want to do a negative entry (then i simply leave my camera on the boat and enjoy playing around in the current). If thats not an option for you then you need to get creative, from a dinghy you could lay down on the inflated tube with your head toward the bow, camera in hand, and roll in, I would however not do a traditional back-roll with a DSLR setup as you might scratch your dome or you wont be able to hold on to your mask (keep your dome-cover on either way).
DSLR, I would never jump in with mine, jump in, let someone pass you down the camera, make sure they know where to hold on or your sync cables might get torn off!
Thats my opinion on the matter, hope it helps!
Edited by MortenHansen, 25 April 2013 - 08:32 AM.