It is very useful to be able to talk with the ship's captain, or whoever's in charge on the bridge. If you don't have marine VHF with a pre-arranged channel to talk to them, then they should have at very least an aeronautical hand-held set which can be kept charged up. Generally hand-helds are okay up to 5 miles away.
Most vessels make at least a little smoke which should give you a good idea of the relative wind. As someone has already stated, the vessel's flag/s can also be good indicators.
Trying to pre-calculate relative wind isn't much use, because the vessel could easily change course before you get there.
Wind "lanes" on the sea surface are at right-angles to the prevailing wind, so you only need to figure out which of two directions the wind comes from. As you approach the deck at say 50 knots (or before your ASI becomes unreliable at low speed) compare it with the groundspeed showing on the GPS. Groundspeed must be lower than or equal to airspeed.
Generally the limits of vessel movements for offshore passenger flights by day are pitch 3 degrees either side of vertical, roll 3 degrees either side of vertical and heave 3 metres total. This is quite a lot and night limits are about half of these.
Have chocks ready for the wheels after landing, unless your machine has skids. Beware if your aircraft has an autopilot and a low-ish main rotor e.g. S76. The autopilot will try to drive the rotor disc tilt to maintain level and this could cut heads off if the vessel is pitching/rolling much. Rather disengage the autopilot if there's no likelihood of needing to lift off quickly and avoid letting pax on/off with rotors running.
Be even more aware of the tail rotor in relation to vessel's aerials, etc which could be moving around. Get some dual training in offshore landings and don't land if you have doubts while attempting to do so.
Last edited by Colibri49; 17th Sep 2012 at 18:06.