Lu got me thinking when he mentioned the rotor disk staying in plane but the fuselage rolling and pitching with the waves causing the swash plate to change it's orientation with respect to the rotor disk. I think he's right that it causes disk movement, but I see it increasing stability and rotor clearance rather than generating any problem.
This discussion only requires a response from the rotor due to a change in swashplate orientation. It doesn't require any mention of gyroscopic precession, aerodynamic precession, Newton's Laws, etc.
Consider a Jet Ranger on floats that rolls left. The rotor disk wants to stay in plane but the swashplate at 3 o'clock (from above) rises. This causes blade pitch increase at 12 o'clock which causes the disk to tilt to a new plane with the tips highest at 9 o'clock and lowest at 3 o'clock.
Since the helicopter rolled left, this new disk position would attempt to roll the helicopter back to level.
Seems to me that this generates stability.
Matthew.