I venture in with trepidation,
heedm: If the helo rotor is at operational RPM and is then rolled to the left by a wave, then I cannot see what would change the blade pitch. The pilot is presumably holding the rotor controls in a neutral position
There will be a ‘tendency’ for precession to occur and may tilt the airframe in the water accordingly. The size and frequency of the waves in this scenario will affect the outcome.
If the wave came from the fore or aft direction then the airframe may tend to tilt left or right, as well as in the direction of the wave tilt. That would be destabilizing.
In your example, the top of the mast and hub-trunnion are going to be putting the main input force into the rotor. The swashplate will follow but not change it’s angle relative to the mast.
That would mean there is no aerodynamic input or result. In fact the swashplate will be working to stop the blade pitch changing.
The only stabilizing effect (that I can see) is the gyroscopic rigidity of the rotor (its resistance to angular change) and that may only help if the helo experienced one wave.
If the rotor is slowing to a stop (as would be the case after a successful autorotation)without a rotor brake in a turbulent sea, then the rotor will start flailing about as it loses aerodynamic control and then move independently of the fuselage.