Last year, Bill Miller spent 210 days cruising, a costly endeavor for most. But not for the retired New Jersey teacher, who received a free cabin (and sometimes more) for a few hours of his time at sea.

Miller is an expert in ocean liner history, and for cruise companies, his knowledge is worth hundreds, if not thousands, of dollars, paid in the form of stateroom, meals, sometimes airfare, maybe transfers and shore excursions. The generosity stems from the cruise lines' need to keep passengers entertained at all hours of the day, especially during those gaps between eating, sunning, sleeping and port visits.

"It's a trade," said Barry Vaudrin, who hosts an Internet radio show on cruising. "You are getting a free cruise for your ability to lecture on a particular topic. It's an easy gig, but not easy to get."

Over the years, cruise lines have been expanding their enrichment programs and theme cruises to retain veteran sailors, attract new passengers and enhance the onboard experience. The enrichment programs, for example, entail lectures of 45 to 60 minutes, often followed by a short Q&A period. The courses fit into two categories: destination, which complements the trip's itinerary and may include talks on the history of the port city, local culture and regional cuisine; and special interest, such as home improvement, forensics, jazz and world affairs. This year, for instance, Cunard's roster of speakers includes a puzzle creator, a BBC war zone correspondent, a crime thriller novelist and a Broadway theater producer.

"You can lecture about photography, technology, arts and crafts, or teach people how to play bridge," said Vaudrin, who has worked as an entertainer and cruise director. "Cruise lines look for people with those kinds of skills."

Discussions are usually scheduled for at-sea days, when the risk of boredom is high. Lecturers often prepare three or four talks, depending on the length of the cruise, but may speak only once or twice. They also mingle with passengers and may be asked unsolicited questions about their specialty in nonacademic settings, such as at a chaise-longue on the pool's edge. "After your first lecture," Miller said, "that's it, you're public property." All in all, though, not a bad day's work for the pleasure of floating around the Caribbean or the Mediterranean.

Theme cruises also offer opportunities to seaworthy educators. Unlike enrichment programs, these special sailings are more concentrated, with one interest taking over the entire ship, such as baseball, knitting or Motley Crue. To satiate the fans, the cruise will hire multiple experts who can speak intelligently on a specific aspect of the same topic.

"You have a lot of people there for a common purpose," said Dan Benedict, a presenter at the Fels Planetarium at the Franklin Institute in Philadelphia and an astronomy lecturer for 10 years on cruises, including Holland America's eclipse-theme cruise in 2006. "There is more pressure to get things right; there are many more critical eyes."