You can try the oil fields in the Gulf of Mexico, and work your way up. For example, southern Bayou Lafourche in Louisiana is practically lined with small utility boat companies that work in the Gulf, usually servicing the oil companies. They frequently hire unskilled deckhands right off the street. (Got to pass a drug test and a physical.)
The pay isn’t too hot and the jobs are often dangerous and unpleasant, but you work until you have accumulated enough ‘sea time’ to go to school, pass a Coast Guard test, and advance to a higher rank. Many young men start out as unskilled deckhands, get licensed as an AB (Able-Bodied Seaman), move on to the bigger boats (work boats) and bigger companies, keep advancing through accumulated sea time, school, and Coast Guard licensing, and either move on to ships or go overseas on an oil field boat. You can also jump over to tug boats and end up on overseas tug boats.
If you can take the danger, the discomfort, the low pay, and all of the crap you’ll be handed, you can advance pretty quickly in the Merchant Marine in the oil fields. I worked as a lowly deckhand for 3 or 4 years, which gave me enough sea time and enough experience to go to Captains School for my 100 ton captain’s license. I became a Merchant Marine officer, 100 ton Near Coastal Master (limited to coastal waters (200 miles)) of the U.S and its inland waters. My license qualified me to operate any passenger carrying motor vessel 100 tons or less, and included tug boats, fishing boats, tour boats, and oil field boats. I also tested for and acquired a Grade B Tankerman’s license so I could pump diesel fuel, etc. After a few years as a captain, I actually had enough sea time to apply for a 1000 ton Master license, but never saw any point in it as I didn’t want to work on the bigger boats.
It’s not hard to get hired (there’s no union in the Gulf marine services) and there are several ways you can go once you get started. Unlimited AB, Mate, Captain, Engineer - heck, you only have to have common sense, decent intelligence, and not be a ‘seasicker’!