Why does it have to be so hard to pick a major?

First thing to realize is, you're not under any deadline and even if you decide on a path now it may look totally different by the time you finish college. I started college as a Mathematics major, switched to English Lit, ended up with minors in Mathematics, French, and almost another in Biology. I've done graduate work in four different fields since then, and have built two successful careers in my life: one as a software consultant and computer scientist, the other as a counselor and life coach. I've never regretted being a generalist. By the way, I'm 60 years old and I'm still learning, taking coursework toward formal certification as a life coach.

Follow your passions and learn what you love (and hate) to do. Both of your current interests are biologically-oriented, so I'd suggest a Biology major in college would not be a bad thing to look into. Get a solid grounding in your college programs in the sciences and math (especially Chemistry), as well as the liberal arts; be sure you can communicate well verbally and in writing. Also be sure to work hard & get the highest GPA you can.

Here's a good page I found on premed planning; if you follow the recommendations for coursework/major I believe you'll be in good shape to apply to either medical school or grad school in marine biology:

Premed Planning - Steps and Timeline for The Process of Becoming a Doctor

And here's a good page on becoming a marine biologist:

Becoming a Marine Biologist

Vasily