Nick, I believe a small uncommanded rotor motion occurs. It doesn't create any problems, instead it makes the helicopter more stable in the water with rotors turning than without.
Ignore what was said about gyroscopic precession. Just consider the swash plate moves with the fuselage and the rotor disk will tend to maintain the plane it is spinning in.
This puts a different relative orientation between swashplate and rotor disk.
This is not psychic, it's physics.
"I have landed on dozens of such rigs, and unfortunately investigated several roll overs that were caused by a combination of the slope of the boat and the wind."
Check my posts, I'm not saying this uncommanded rotor movement is causing problems. I say it's small and it slightly helps matters.
Matthew.