The cruise lines understandably get hundreds of requests to be guest speakers, and getting through to the "right people" is hard. That is why they use companies like sixthstar.
I can't say which companies hire their own because I don't keep track, but if I had to guess I would guess Princess and Holland America. A lot od cruise lines don't even offer them.
In any case, if you don't already have a track record as public speaker, and can't provide visuals and a video of your presentation don't even bother. There are too many people out who say they know what they are doing but they really don't.
The best way to get in is to be recommended by someone who has already done it and knows the ropes.
I suggest you take this topic over to our "crewmembers" message boards for better ideas.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.