You can use something like Khan Academy for free online to learn the math. A one on one live math tutor might be good too. There are retired people that volunteer to teach. There are plenty of high school and college students, not to mention teachers that will tutor in math for a reasonable fee. Much cheaper than a for credit college course.
If you approach learning math, and going to the academy, like a job, and simply work at it, you will do much better than most.
As an old hawespiper, I say: Go to the academy!
If you can take “STCW” Basic Training” for free and get work as an OS, that would be a good thing to do, while you prepare for the academy. Some jobs do not leave much opportunity for studying, other job provide a lot of time to study. Many mariners are good teachers.