If you still want to be a marine biologist, the only thing standing in the way of you making it happen is you.
When I was in grade 2 I wanted to race motorcycles. In grade 3 I wanted to be just like my dad, even though I wasn't sure what he did at the time -- turns out, electrician, though he isn't one now. In grade 4 I wanted to play baseball. In grade 5 I wanted to write. In grade 6 I wanted to save the environment.
It's changed a bit since then... I had wanted to own my own bar for a while, when I was 16 or 17 or so, and after that I had wanted to become a historian for Parks Canada for a while... but now, since I'm in English and taking philosophy, I can look to the future and say 'writing is possible' and (if I pursue law) 'saving the environment is feasible' or given the fact that with enough capital anyone can start a business (plus I have experience in the service industry) I could become 'owner of my own bar'.
Now, I may end up not doing any of those things, because something else might pique my interest. But really, I'm only in the position I am right now because it's along the path of where I want to be. If you don't feel that you're on the path that you want to be on, there's always time to forge a new path towards your goal of Marine Biology. It'll be hard work, maybe even hellish work, but if you want to be a marine biologist and are upset that it didn't happen, that's the only way to do what you want -- I mean, noone's going to drop a diploma in your lap and throw you in the pond.