As with hotels, there are international corporations and small independents. Here's a taste of both sides.
Jennifer Murphy, 28, bartender, Cunard Line
How do new crew acclimatise?
We all get a four-week induction and familiarisation course. We get on-board training and learn about ship duties and safety. If you want to take NVQ courses, there are NVQ assessors on board.
What's life on board like? There's no doubt it's different from working on land. Your hours can seem longer, and you have to work seven days a week for stretches of up to four or five months. But you have no commuting time, plus all your guests are having a great time. The teamwork between you and your colleagues is second to none. You're never alone. There's always something going on for your entertainment, including shore excursions, crew shows, spa nights and, of course, the parties in the crew bar.
Do you get homesick? It's difficult leaving family and friends at home. But I have more friends on board - which includes guests. In fact, I miss it when I'm on leave. There aren't many jobs that can provide such satisfaction, take you around the world and give you the chance to save all those dollars.
How does the contract work? I do a four-month contract followed by one month's vacation. We do a seven-day week and, on average, 10-hour days. To provide optimum service we work split shifts, which is perfect as you can go ashore or sunbathe in the afternoon.
What's your ambition? In my mid-teens I started work in a local hotel bar. I never expected it to take me on board the world's most famous ocean liners, Queen Elizabeth 2 and Queen Mary 2 - or to do a world cruise. In the future I'd like to become a cruise sales specialist and learn other languages.
Amy Boothman, 24, assistant purser, MV Hebridean Spirit
What qualifications did you need? I've got a GNVQ Advanced in hospitality and catering at distinction level, plus I had spent two years at the four-star Victoria hotel in Sidmouth, Devon - one year was spent on a trainee management scheme and the second year as a duty manager.
Have you always been a purser? No, my first job on board was on the Hebridean Princess as an accommodation stewardess. At the time it was a little less ambitious than I was planning, but I decided to take it as it was a good way to get into the company and prove myself.
How does the contract work? My contract is eight weeks on followed by four weeks off. I always look forward to going home, but in all honesty I also look forward to returning to the ship. We're often in two, maybe three different ports a day. At the moment we're in the Seychelles, so I've been to some of the most amazing beaches.
What's the atmosphere on board like? The Hebridean Spirit is such a small ship, carrying a maximum of 80 and a crew total of about 70, so we often say we're one big happy family. But like any group of people who live, work and socialise together we have our moments. Then again, because it's such a small ship, problems are usually ironed out quickly.
How does your job work? I'm employed as assistant purser, but I spend my watches either as the accommodation manager or the purser. As purser, my job includes managing reception staff, looking after the money on board, paying salaries, dealing with local agents and port authorities clearing the ship through immigration, customs and port health and ensuring the requirements of the country's laws and regulations are met.
As accommodation manager, my job includes managing the cabin stewardesses, housekeeping utilities and the laundry boys, checking public areas and cabins at least twice a day - and ensuring officers' cabins are serviced and well kept.