When was it decided that every cruise must have a theme?
Food and wine, wellness, photography, comedy, science ... think up a theme and it probably exists.
There are political cruises, religious cruises, gambling cruises, even cruises for cigar lovers.
One of the most unusual I have seen is a runners' cruise, where you complete the equivalent of a marathon during the journey instead of piling on the kilograms at the buffet.
Among those joining the ranks this year is Oceania Cruises, which is offered themed sailings for the first time. The "signature sailings" feature across 10 European voyages, themed around food, wine, music and health and wellbeing.
There will be guest performers or presenters on each voyage and activities will include seminars, cooking classes, wine tastings and concerts.
Oceania says groups with shared interests have often cruised together in the past and the theme cruises will create a further opportunity for like-minded travellers to cruise together.
MSC Cruises is currently operating a "diet cruise" in the Mediterranean. The cruise is being led by French weight loss expert Dr Pierre Dukan, creator of the popular Dukan diet, and was marketed as a way to "shed kilos, tone up and tan while cruising the Med".
Those on the cruise are getting a personalised diet and training programme, attending daily workshops and doing on-shore walking tours to help get rid of the kilograms.
If it doesn't sound like much of a holiday, MSC says it is all about losing weight naturally, so participants are still having fun and "enjoying great food".
Celebrity Cruises is also targeting the health-conscious, although it has opted for themed activities on regular cruises rather than dedicating whole sailings.
Celebrity's offering is based around the best-selling Eat This, Not That! book series and includes interactive trivia games to test guests' knowledge on healthy eating options.
Are we no longer content to just sit by the pool and drink cocktails?
Are cruise companies battling to find a point of difference in this burgeoning sector of the travel market?
Or is there some other reason for these themed offerings popping up all over the place?
The general manager of the International Cruise Council Australasia, Brett Jardine, believes cruise companies are looking for unique selling points.
"As capacity grows, they're certainly going to need to be sharper with their marketing to create a point of difference," he says.
Jardine believes it is also about convincing non-cruisers to give it a try.
It is not so much about filling a particular departure but about getting people to try cruising once, in the hope that they will be converted (cruising has a very high level of repeat business).
"If you look at the themed cruises, they are alerting people who haven't cruised before to what cruises have to offer in terms of on-board activity and entertainment," Jardine says.
"A potential objection to cruising is that it's boring and there's nothing to do, so they are appealing to an audience who are fans of that particular special interest but who are not necessarily cruisers.
"Cruise lines are looking to attract more passengers ... it's an investment in the future."
In some cases, themed cruises are simply conventions at sea, with a ship replacing a hotel or other venue.
In April Carnival's Carnival Paradise will play host to what is claimed to be the first horror convention at sea.
The vessel will set sail from Florida with hundreds of zombies and other horror fans on board, for the four-day "Tides of Torture" convention.
In other cases, themed cruises are simply able to offer a safe and reasonably private environment for "alternative lifestyles" or activities that would normally be limited to specialist retreats.
Dedicated departures of cruises ensure that there are no other passengers to get offended or to sit in judgment.
There are many nudist cruises on offer around the world, along with swingers' cruises with erotic theme parties.
Many gay, lesbian, bisexual and transgender travellers also opt for cruises, whether it is for the opportunity to meet people or simply to travel without encountering stigma.
Pick an interest
If you have a special interest and want to find a cruise to match, try the website themecruisefinder.com.
The site lists themed cruises under categories such as "wellness", "golf" and "intellectual pursuits" and provides details of individual cruises as well as links to the cruise lines or agencies promoting them.