Breaking into the merchant marine is a catch 22 in the USA. You can't get on a ship until you are in a union and you can't get into a union until you have a job! You will need a letter of commitment from a shipping company to begin and for me at least, it took many days and a lot of shoe leather to secure one.
If you could enter a merchant marine academy, after graduation you will be licensed and securing a position will be much easier. Keep in mind, that like many other occupations today, automation and costs are cutting the work force and there will be fewer and fewer jobs available. It has been a grand occupation for me and I have traveled the world on someone else's nickle, AND been paid to do it.
"Any a**hole can make a boat go; it takes a sailor to stop one"
Spike Africa, aboard the Schooner Wanderer
, Sausalito, Ca. 1964