I'm going to go a different route and say make sure to take English, speech, and communication classes. At least for me, (BA in biology and BS in fisheries and wildlife) I went to school with and work/ed with some really smart biologists. Many of them couldn't write, spell, or speak publicly to save their ass. Everyone has to jump through the same biology, chemistry, and calculus classes but students who could write and speak well really stood out and it's a HUGE part of being hired these days. I don't care how amazing your work or research is, if you can't communicate it effectively it's worthless.
I'm lucky to be working in the field right now doing fish studies but we recently gave presentations to visiting Australian officials after winning some international restoration awards and they don't see our fieldwork, they see the presentations and dissemination of our data.
So yes, chemistry, biology, and math are important for any biologist. But you can set yourself apart with your communication skills.
Also, what do most "marine biologists" end up doing? I know one who bartends, one who teaches, one who serves food, and one who's sailing around the world doing research. It's a tough field to get into. Like Matt says, environmental engineering is taking off.