I've been on a number of cruises and, of course, saw all the photography going on. A couple times I grabbed the photographers and asked "How does all this work?" I think that's what you're asking? Here's what I found out.
Most of the non-navigational services (shops, health & beauty, photography, etc) are provided under contract. Many of these contract companies span multiple cruise ships and multiple cruise lines. Almost all of the contract companies are European. Some hire Americans. The hires are mostly out of Miami.
The contract photography provides mini-lab processing services on board, as well as taking and selling images. As a photographer, you take a bunch or pictures and then spend lots of time down in the mini-lab processing prints.
Like all of the contract folks, you are also "filler" for the cruise director. You have to have everything from shorts to a tux and go to the parties you're not photographing. I've talked to a number of the folks that work the cruise ships -- it can be fun, but you have half a cramped cabin well below decks. It's kind of a group camaraderie thing.
If you're thinking about bidding a contract -- that's the only way to make real money. Although doing it for a six months or a year is fun. (The individual contracts are usually six months.)
If you have specific questions, shoot me an e-mail.