I've worked on a cruiseship as personal trainer for a year, it was amazing but very intense, very challanging as well..
It is great opportunity, but it is def. not for everybody. Also I had some ethics issues, that is my reason why i am not returning. I've written my entire experience down in a blog; cruiseship-personaltrainer.com
Unlike some others, I have finished my contract and can give you a unbiased and honest opinion of the company and the work. Because there is a lot of false information on different websites and from different people.
There is a lot of anger from people that did not finish the training or the contract, and also a lot of extremly positive information from people that see everything trough pink glasses or never ran into trouble, or just never had many problems with ethical issues.