There are some things you need to do before you consider getting a job on a ship.. Yes! they are alot of fun and can be dangerous but hey thats with all things.. The first thing you need to do is to contect the US Coast Guard for a Merchant Mariner Packet!.. which is free! then you have to fill it out, you must get a coast guard approved physical and a DOT five panle drug test, which you need to pass before you get what is called a Transportation Worker Identification Credential or TWIC card for short..
it will take about a month over all to get this info if your not wanted or have any federal crimes against you. the pysical will run you about 25 - 50 dollars depending where you live. and the twic is $127.75... then you send in everthing to the Coast guard and proof of payment for the MMC which is 140... I know all of this because i have just gotten out of doing this and now im just waiting on my MMC to come back. it sounds like alot but in truth its worth it..
Good luck and may the metal be with ya!