From my experience as a shipbroker, tide should not be a problem, as the roro ship in question will probably be in the enclosed dock, with the water level constant and the ramp will not be too steep. Assuming that the height of the helicopter allows, the aircraft will probably be lifted first onto a MAFI trailer (a low specialised wheeled platform) and then hauled into the ship by a tractor vehicle. Much safer that way. Only if the height of the helicopter plus the height of the MAFI trailer is too much for the height of the ship entry, would they consider moving an aircraft into a ship on its own wheels.