My experience has been that ship's libraries are not staffed on a full-time basis. It is often a duty covered by the internet staff (which is often one person).
Obviously since I am "into" computers, I'd prefer to be the internet manager on a ship. The cruise director's staff is the place to be since you often will have better quarters and duty schedules. But bear in mind that collateral duties also come with any job on a ship. Examples are being assigned as a muster station leader, or to the crew of a lifeboat. Or you may have to assist in getting tours organized and sent off on time. For instance, on the Freedom Of The Seas the cruise director (Ken Rush) told us he was qualified to command a lifeboat and that was his emergency station.