I have to agree with Dr. Bill that not alot of institutions offer undergraduate degrees in Marine Biology, but rather offer a general biology degree. When I earned my undergraduate degree, I enrolled in classes that dealt mostly with the marine, estuarine and limnological sciences and supporting classes such as invertebrate zoology. When I went to graduate school I became more specialized in a field (marine chemistry and toxicology) under a general Environmental Chemistry degree. Not exactly what I had originally planned on doing, but I found I had an affinity for chemistry and biochemistry and was able to combine that with working in the marine environment.
What you will probably end up doing when you start your education is slowly find a niche within the wide realm of (marine) biology that you like, and specialize in that field when you continue on in grad school.
Look at the course listings available for different undergrad schools and pick one that offers the classes that will stear you in the direction you want to go. Make sure it is a research intensive institution with very good lab facilities and a highly rated science department.