Okay, let's have a return to sanity here...
"The term "Merchant Marine" has a common meaning, specifically referring to the civil merchant shipping systems in the US, and UK, and the rest of the Commonwealth. "
Yeah, they don't exist in the Third Imperium - thus, we considered the term 'free'. You won't have to go many light years to find this a common practice in science fiction. If this were a 2300AD book, the argument might carry some weight, but this far ahead? It really can be whatever we (and you!) want it to be.
I can guess what the immediate response to this will be, so let me first say that if you look through Mongoose's catalogue, you will see that we are not completely without knowledge of things Naval.
"There are job titles used today for people doing different jobs now than the people the job title was named for. "
This.
"It's no Mongoose GTFT equivalent, anyway. "
That is absolutely _not_ the purpose of this book. Why would it be?
"The "how do you know what it'll be in the XXth century argument" doesn't apply."
Of course it does. Again, it is a fairly common approach in science fiction.
"did we really need another military/paramilitary career in Traveller, in a trade-themed book? "
Why not? For the sake of a tiny amount of page space, why not?
These books are intended to be toolkits, from which you can pick up the bits you like for your campaign, and leave the others for another day and another task. You don't really think that everything in Psion is intended to be shore-horned into the same campaign? Merchant Prince is no different.
In theory, there should perhaps be combat-related careers in every similar book to allow us to cover the players who like to use them, across every campaign type.
"To make matters worse it's not the first blunder of the Author. "
Right.
The gentleman concerned has written literally _thousands_ of published pages. You think you could do that amount of work without having two things crop up that people do not agree with? If so, send me your CV. We'll talk.
"An actual merchant marine career would not only have extended Traveller nicely,"
Good idea, write it up and send it in to S&P.