Without knowing the specific details of the two programs at your school, I think that either degree would actually provide you with a great deal of flexibility. Whichever one you pick, you may be able to choose electives which will give you additional competencies. For example I decided to do my school's cell bio emphasis rather than cell & molecular biology & biochemistry, but I'm also taking extra chemistry classes; in essence, I'm making the cell program a little more like the cell/molec/biochem emphasis and trying to find a balance that suits my interests.
What you should do is look at the specific classes offered at your school, the similarities and differences between the two degree programs, and the requirements of various graduate programs in marine biology and other areas that you are considering. "Aquatic life" is a pretty broad topic and depending on what you want to study, a different knowledge base and skill set will be required. A detailed understanding of inorganic chemistry and geology will be critical for some areas of oceanography or marine biology. Subjects like botany, microbiology and community ecology may have varying degrees of utility depending on your specific interests.
I hope this doesn't seem like a non-answer, but the truth is that either degree program could prepare you for a variety of careers and advanced studies in biology. And anything that you learn from one area of biology can be applied to another. A zoologist's understanding of animal physiology might be useful to a disease researcher, and a microbiologist's understanding of unicellular organisms can be applied to multicellular animals (in fact studies on yeast and E. coli have helped answer many questions in human biology). Bacteria, Archaea and unicellular fungi and protists are found in—and essential to—all ecosystems, including aquatic ones and to studies of human disease, so courses in microbiology would be useful to you in either field.
Most undergraduate majors in the life sciences are built on a common core of coursework in foundational topics like ecology, evolution, cellular and molecular biology, biodiversity, etc. and supporting classes in chemistry, math and other fields. At many schools, you'll be able to round out your major with electives that expand your training. Your choice will depend on how specialized the various programs at your school are. If you're not sure what you want to do, I would recommend going for the less specialized major, which in this case sounds like it would be the general biology program. Since you are a freshman, I recommend taking as many of the courses that these two majors have in common as possible. Keep exploring biology and choice might become more clear to you. Talk to professors and other students, and people from other schools that have graduate programs in zoology, marine biology, microbiology and other areas that you might be interested in. See if your school hosts seminars where scientists come and talk about their research and this might give you a better idea of what you're interested in. At my university, the chemistry and biology departments both host weekly seminars, and many of these are on topics in biochemistry and molecular biology, at the interface of the two disciplines, and attending these has helped me see what I'm really interested in, and what my strengths are.
