How can I get work as a guest lecturer on a cruise ship in exchange for travel?

I am a Licensed Psychologist and Certified Coach. I have lots of experience presenting on a variety of self-improvement and parenting topics.

So far in my experience on a cruise, I've not seen guest psychologist lecturers. I have seen many people on board providing lectures to guests while on board. I'm not sure if they were allowed to go for free, but they brought their families as well, so I'm going to assume at the very least they were offered a really discounted rate.

I know lines like Holland America and a few other lines that offer cruises for seniors have many guest speakers quite often. But again, I'm not sure what they would have avilable for you.

A good thing to do might be to call the cruise lines directly to talk to them. They should be able to transfer you to someone that should be able to help.

I have cruised many many times and have never seen a psychologist who was the guest lecturer for passengers. I have a friend of mine who teaches how to use the internet aboard cruise ships and he has brought his wife along on a few trips. He cruised the Panama Canal and from NYC to England a few times.
My advice is to learn a good skill that would be entertaining to people who are on vacation. NOBODY wants to be psychoanalyzed while they are on vacation. People want to forget their problems when they have time off from work.