Qualifications
If your master plan is to head down to the docks and wave around your liberal arts degree, hoping your stellar 4.0 GPA in art history will be enough to get you on board, you might want to go back to the drawing board.
The only degree a captain will be interested in seeing is one from a maritime academy, and even then, you better also have the skills to back it up. Looking for a second piece of paper to wave? A Merchant Mariner Credential certainly wouldn't hurt.
If you're not interested in a formal education, you'll have to prove your worth through hard work. A captain might add you to his crew in order for you to learn on the job, but don't expect much pay (if any at all). Just settle in and get ready for some serious manual labor as you learn the ropes. If you can survive doing that, you may have what it takes to earn your living on the high seas.
Don't forget, you'll also need to keep your social skills in check if you hope to make it as a part of the crew. That means your ego needs to take a long walk off a short plank. This is strictly a career for team players and folks who thrive working in groups.