Lu, when you first posted to this thread, you asserted that the helicopter's response to wave action caused a rotor response that is 90 degrees out and generates instability. I showed you that it is 180 degrees out and serves to reduce the motion caused by the waves. You then responded to me by suggesting I was only looking at a snapshot in time. Not so.
I isolated one movement but just as easily could have generalized to any deviation of the helicopter's vertical axis. Combining pitching with the rolling may confuse you, but to the helicopter it is still just a movement away from the vertical axis. As Nick mentioned, the roll rates that the helicopter can generate are much greater than those experienced due to wave action. The rotor won't lag in it's "correcting" position, it will respond to each and every deviation of the helicopter's vertical axis.
Stability is when a system that is disturbed tends to return to it's state prior to the disturbance. What I have described IS stability.
I don't doubt that what you have told us about the icebreakers and some earlier floats is true. I just don't think you've found an appropriate explanation for it. Everything you've said wrt the icebreaker sounds like the natural rebounding frequency of the float was resonant with either the helicopter's vibrations or with the deck motion (I assume the former).
As far as floats vs hulls, the vertical c of g you mentioned is a small part of the story. What's important is how the center of buoyancy moves when the helicopter pitches and/or rolls.
Nick, I think you misunderstood Lu's initial assertion, which I believe is valid. The swashplate's orientation doesn't change with respect to the servos or the fuselage. It does change with respect to the earth, as the fuselage moves. The rotor disk does not want to move when the fuselage rolls or pitches due to it's angular momentum (make this easy and consider either a teetering or a fully articulated rotor). Thus the rotor disk and the swash plate change their orientation with respect to each other....same effect as putting in a control input, but without any change in the orientation of the flight controls with respect to the pilot.
Thanks for the warnings wrt Lu, but you will find I tend to stick to the discussion rather than getting personal.
Matthew.