I would have thought it'd be more profitable just to make them go to a movie theatre when ever they were at port, as it was my dad got to see a bunch of movies (including three James Bond movies, and two starwars movies) for free?
What your dad was experiencing was a "non-theatrical distribution" network. While he was watching the movie for free it is probable that his shipping line was paying a much reduced fee to some non-profit group to get access to the movies and have the film reels passed around within the network and it may have been a condition that no entry fee be charged.
Non-theatrical releases were regularly made through limited groups, often non-profit such as educational institutions, hospitals, military bases, nursing homes, film societies etc. on condition that they not compete with theatrical releases. Cargo ships at sea would fall nicely within this group and they could exchange film reels in port.
From the movie distributors point of view this is differential pricing. They have identified a market where they would probably make very few, if any, normal sales and are able to get some return that they would not otherwise receive, by charging a low price to the non-theatrical distributor while placing stringent conditions on the showings.