We do occasional tanker support using an Astar, and its not too traumatic.......
1. The shipping company or agent will provide ship's details on request beforehand, to determine the deck setup - whether the landing is to port or starboard etc. Usually the helidecks are on the forward third of the ship.
2. Marine VHF is very helpful. Call up to determine winds across the deck, and how much, if any, of pitch, roll and heave the ship is experiencing.
3. You can request the ship turn to a heading that will allow you to make an into the wind landing.
4. Typically, the approach procedure is to circle the ship, and make your finals from astern at an oblique angle (in case you need to go around). In the case of skidded helicopters, remember that the deck itself may be sloped or angled, and sometimes slippery, so beware of ground resonance or sliding across it.
5. Your ships crew, if there is an approved helideck, should be well trained in helicopter movements - our company does like to take one of our own trained ground crewmen whenever we can.
6. For departure, pull to a high hover, if possible back away and align the helicopter with the direction of ships travel, which will mean accepting a right or left quartering wind, and depart over the water, keeping away from the ship. (Captains aren't very keen on take offs over their decks or superstructures).
Some other points - Personally, I like to plan my fuel very carefully, as I like to have as much available power as possible, especially in a single.
There may be lots of bollards, stanchions, pipes and rails which can make you slightly uncomfortable in a good wind, but just keep your eyes open.
If the ship is under way, there maybe a brief visual clue transition from a stationary to a moving horizon.
If you have to be sat on deck for a while, and there is a noticeable swell, there is a chance that you may get seasick - this happened to me once.
Hope this helps, and good luck!!