Until the winch man is on board, you take guidance from the pilot of the helicopter.
Once the winchman is on board you will take guidance from him.
Obviously, the master of the vessel is responsible for advising helicopter crew of navigational issues which may affect the vessel such as shallow water, tidal flows etc.
Basically, they are there to help the casualty, not take charge of the vessel. It is in your interest to do as they say unless there is good reason to question it, but you must make them aware if there is a potential problem., ie shallow water.
Quite simply, take guidance and work with them. You are in a strange circumstance, they train for it and have experience.
If this is a legal issue leading to a possible claim, (which would be a damn stupid idea anyway) then, do not use the above comments. These are just general guidlines we use in the RNLI, and hopefully re-inforce common sense.
I don't know for real but Rival Spirit has been used for practice runs in the past and the heli-pilot tells you precisely what to do, via Ch 16. Usually to go onto port tack. cos the pilot sits on the starboard side of the Wessex and he doesn't want your mast in his face when manouvering. Then and to proceed in a straight line at about a speed that you are comfortable with, in our case 5-6 knots, it is easier to keep the heli and boat in sync if both are making way. The diver chappie then comes on board and tells you precisely what they want you to do but does consult the skip as to what is and isn't feasible for your craft. So, whilst the chopper runs the thing I felt that both sides contributed to the overall rescue plan.
BTW if you want to get invovled in the practice runs, contact to MCA and they'll book you in. We found it to be a really interesting exercise.
A further hint is that during the crucial moments of the winchamn being lowered down to you, do not make any radio calls to the pilot. His is 'in the zone' during this maneouvre and radio chatter is not welcome nor will it be responded to unless directly related to an emergency the winchman.