There are a number of consultancies dedicated to placing registered members with cruise ship companies. Some are free of charge, while others charge a one-off fee for registering with them. Before signing up, always make sure you know how much you are liable for if you gain employment through the agency.
Some agencies deal only with managerial roles. Others focus on particular areas of cruise ship employment, such as entertainment.
You can also apply via agencies or employers in other countries - the US is clearly a major source of employment - but remember that if called for interview, you will have to make yourself available overseas.