For those cruise lines that don't use an agency, I would send a videotape and introduction packet to the line's home office, directed to the director of onboard entertainment.
But before I even did that, I would first do my homework. Is your topic relevant to a cruise ship audience? Is it the kind of topic that has been presented onboard in the past, and been well-received?
Don't forget, you might land your first job without doing this homework, but you could shaft yourself in getting future bookings if your topic doesn't go over well, and you get bad passenger ratings on your talks. Chances are the cruise line will never use you again after that, and you'll never get the opportunity to "polish" your presentation for future cruises.
I would check out SixthStar's website and the guidelines they list for onboard lecturers. There's a lot of good information there even if the particular cruise line you're interested in doesn't use them.