Long haul: Joined MSC as AB Limited at 58yo. Retired from gov't service 5 yrs later at 62 as AB Unlimited. My detailer OK'd me to update the STCW training after I signed off my last ship, so that's valid until 2018. Left MSC with >100K in my TSP (gov't 401k) and about half that in my checking acct, all the while paying rent, car ins. and all other bills when away at sea. My health ins is paid by the gov't for life. Still sailing. You sound like a young guy with good goals, so YMMV.
There are good ships and better ships. If you want to have a blast and see ports that no other vessels visit, work with Navy divers, both stand watch and do day work with lots of OT get yourself on an ARS. Watch out for Cpt. Peter Long. He's a dick, but he'll respect you if you do a good job aboard.