Not being a well trained diver, but an old artifact with (likely) some ingrained bad habits, I had to think about what I do with my hands when I dive.
The best I could come up with:
1) Descents . If there is no current and the vis is good I free descend with both hands tucked gently, but very close to my body. I usually hook my thumbs or let on hand gently rest on top of the other. When I'm carrying my camera, I cradle it against my torso using both hands, with one cradling it like a baby (hey, I LOVE my camera).
In a stiff current, I hold my camera in my left hand along my side, and hold my right hand (gloved) forward and slide it or pull along the side or down (mooring/anchor) line. Without the camera, I simply hold my free hand along my side.
2) On the bottom, I hold my hands, thumbs locked, in front of me, gently hanging down slightly, unless I am actively swimming in a current. Then, I just tuck them more tightly. With the camera, I either tuck the camera "in" like on the descent, until I find a subject, or I swim with my hands holding the camera against my chest, but "aimed" out, so I can put it to my mask and frame fairly quickly. This is not efficient, but it allows me to check camera setting, then frame, in one motion. I only do this in light to no current conditions.
3) When "hovering" around, which I like to do, I lock both thumbs, but lower on my, ahem, gut. I hold the camera with four fingers of my hand, but I must admit I just let it dangle loosely i my fingers and don't bother to tuck it in...and, yes, i have it on a wrist lanyard. On those moments, I am focusing on relaxing and admittedly my hands sometimes just "hang" in front of my body.
4) Ascents with camera or other junk in my hands require that I hold my hands higher around my chest. With camera tucked in my left hand, I unclip my guage console and "hang" it over my forearm. This leaves my right hand free to grab the hang line if the current is trying to sweep me away, and to hold a light, if it is night. I find it very easy to monitor my computer and pressure guage this way. This is probably a poor method, but I feel more comfortable with fewer "hand swaps" necessary when getting read to make a stop. I've watched someone drop their camera once, with 400' of water under the boat, because they forgot that it was hanging on a wrist lanyard and swapped hands, letting the critical hand dangle just enough......it almost made me cry (if it were my camera, I WOULD have cried.)
Anyway, I likely have some bad habits, but "dat's da way I do it"....