Just wanted to share my experience with recreational charters/boat.
After reading here about hooking the left tank on the boat and the right one in the water, I proceeded to do just that, asking the DM to hand me the right tank after I jumped in. It was not easy, not at all, at least for me (and I'm a strong muscular man and have no problems getting them on while kneeling on the bottom). After I clipped the neck of the tank to my shoulder D-ring, which in itself was hard as the tank was very heavy and I was afraid I would drop it, I tried to clip the bottom of the tank to my butt rail. Impossible. The wing was inflated to keep me from sinking and tended to push me face down in the water and the heavy steel tank clipped to my Dring was pulling me down. The other tank was pulling me to my left so the net effect of all these forces was that my right butt rail was practically the highest point, no way I could've raised the tank all the way there, let alone clipping it. The DM had to jump in the water and she clipped it for me, and from there no more problems.
The second dive I chose to clip both tanks while on the boat, no more problems whatsoever.
Coming up I just went up the ladder with both tanks by my side, not a problem. It was a bit tricky to unclip them while on the boat as the space was tight, but once I was able to sit down it was easy.
So, for me, there will only be one way of doing it from boats: both tanks on while topside then splash.