hehe.... a simple question with a complex answer.
The licenses you need to get are the 3d Mate, 2d Mate, 1st Mate, Master.
The differences between the licenses are basically time, experience, and knowledge as far as I know. You get these by taking the Coast Guard exam.
And within these licenses there are categories, very similar to type ratings in the aircraft, but they're not tied to a specific ship or class of ship, but rather tonnage. 50 ton, 100 ton, 150ton, 200ton, unlimited., etc... these ratings determine what type of ship you can work on. To find employment with a shipping company to work on a cruise liner or any other large commercial ship, you'll need a minimum of an unlimited 3d Mates license, since there probably aren't any 200 ton cruise liners out there. You don't have to get a 50ton rating before you get an unlimited rating, you just go straight for the unlimited. But you do need the 3d mate, before you go for 2d mate, before you go for 1st mate and ultimately master.
To get the time to qualify for the 3d Mate's license you can go to one of the maritime academies (you'll graduate with an unlimited 3d Mate's and a degree), or you can start at the school of hard knocks working as an able-bodied seaman and going up from there.
Talking with some mariners out there, cruiseliner work is actually the low end of the pay scale, with the more lucrative jobs being on cargo ships (Ro-Ros, container ships, bulk break ships, tankers, LPG ships, etc). Plus you get better schedules working cargo ships.
There's two different tracks you can take either the deck track or the engineer track. Deck track - you're up topside, driving the ship, you worry about proper loading (stability is critical), dealing with cargo ops, etc. Engineer track - you work down in the engineroom, and you just keep the engine running, and keep the lights on and the fresh water running. Usually you don't switch between the two (unless you are willing to start all over again.)
As far as the military side goes, time and experience does not transfer at all. A military pilot can take a military competence test to get the appropriate FAA license. and boom all his time transfers, he becomes competitive with his civilian counterparts when looking for employment at any airline if he has the right kind of time, e.g. a helo pilot probably can't find work at United, American, etc.
For a Navy officer after 20+ year career at sea driving everything from frigates, cruisers, destroyers, and maybe command of an aircraft carrier or amphibious assault ship, none of that sea time is creditable and if that officer wants to look for employment in the civilian maritime industry, he has to take the 3d mate's exam and start working from there and time building from there.
I think the Coast Guard does things differently, their officers do get credit for their time at sea.
Just my speculation here, I think the reason why Navy officers don't get credit for their sea time in the civilian sector is because while the Navy does drive ships, it's not their primary job. No one else drives the ship, but the primary job of a naval officer is to fight the ship/tactics and weapons employment and while there is time devoted to driving ships and the ins and outs of the propulsion plant, there isn't any attention paid to cargo loading, etc.
Woodreau / KMVL
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.