The moving vessel will create its own wind which can be markedly different from the real surface wind - anything on the vessel (smoke, flags, windsock etc) will give a good indication of this relative wind - this is the one that you want to be pointing into for the landing if possible. The superstructure of the vessel can cause a lot of turbulence which can be very strong so beware - but most tankers seem to have the superstructure at the aft end so it shouldn't affect a HLS mounted amidships.
The vessel can pitch, roll and heave (vertical movement), the bigger the sea the worse it gets and the heave is the one which might catch you out as the deck rises to meet you.