There are definitely pros and cons for using a staffing agency to get a cruise line job. For one thing, going through the agency may be the only way to get a job with some cruise lines. If the cruise line has contracted with an agency for all culinary staff, and that's the department you want to work in, it won't do you any good to apply directly to the cruise line. The cruise line will simply forward your resume to the agency anyway.
Some staffing agencies also provide you with assistance that can get you hired - they help you update your resume, improve your interviewing skills, and more.
Remember: most legitimate agencies DO NOT charge fees for their services. They usually get paid by the cruise lines for whom they are recruiting. You really need to get as much information as possible before you apply with an agency.