Landing On Deck
First of all, you have to find the ship! Once they've got full steam ahead, Captains are very reluctant to slow down for anybody, so be prepared to look for your target a fair bit away from where you (or the owners) think it should be. However, they may turn into wind a little bit, if you ask them nicely.
Aside from knowing where the deck is, you will need a radio frequency to talk on, either the customer's own or a general marine channel. This is not only so you can ask where the heck they are, but so they can give you the wind and how much the deck is going up or down (that is, pitching and heaving, for the technical).
Flying around vessels is much like doing it round mountains or buildings, especially the latter, which are more slab-sided. The wind does strange things round them all, so have the normal escape route handy, by approaching obliquely. There will be turbulence in the lee (and elsewhere), so approach to the windward side, getting into the hover before you position for landing as the highest point of the swell is reached, not when the boat is rising - use your peripheral vision to keep an eye on the deck, and focus on the horizon, as normal. Look out for ground resonance, and a slippery deck, which will likely be sloping as well. Stabilisers will compensate for deck movement, so turn them off once you are safely down.

When leaving, align yourself with the way the ship is going (even if there is a crosswind), and avoid the superstructures on your way out.

Fuel
Helicopters don't need aerodromes, and minimum fuel figures reflect this. However, they are calculated for level aircraft. Odd attitudes, say when slinging, may cause a fuel boost pump to become uncovered and give you a nasty surprise just when you don’t want it. On a 206, the unuseable fuel after a boost pump failure can be up to 10 US gals, which is uncomfortably close to the minimum figures below.