Once in a while I see people getting priority in airport queues when they show their Seaman's book, which is a record of their shipping career. I also was told on different occasions that people flying on a Seaman's book get better fares on one-way trips. Is this true, or is it one of those urban legends?
AFAIK, it does not work that way, it is not a type of discount that you get at a counter by showing a seaman's book or ID.
These kind of discounts are usually given to seamen by their companies (ship owners), the company will contact the airline and get a special discount. This is used when the shipping company needs to move crew from one area to another, to operate a different ship or when moving seamen back to their ships after leaves.
One more thing, this is usually done by arranging bilateral agreements between shipping companies and airlines, it is not a guaranteed benefit.
This kind of agreements do exist not only with ships companies, but also it can happen with major companies, I have seen oil companies with such benefits, but again, it is not something the employee can get by himself, it has to be done at company level.