Besides the classes you have to take towards Marine Biology in college, what are some extracurricular things that would help towards getting my degree in marine biology?

Zoos often have summer internships that you can apply for, and sometimes they are even for working with a specific set of animals, like marine animals. I would look online to see if your local zoo offers anything like this, or contact them directly (not all internships are posted online). You might be able to do this while still in high school.
You might also look into research positions in the biology department when you get to college. Sometimes professors are looking for research assistants for their own projects, so you might be able to find a professor whose interests align with yours. Again, you want to reach out to them directly if you can't find anything, as they may have a need that they are not posting online (or know someone who does). Just send them an email or set up an in-person meeting where you talk about your interests and how you see them matching the professor's, and asking if there is any way you can get involved. Most professors will love to talk to you about this, even if they don't have any current openings - just make sure to be very polite.