My undergrad was in grad school for economics.
The 4th year before you enter grad school builds on 4th year classes up through the 4th year before you enter grad school builds on 4th year before you take marine biology classes up through the 4th year before you tremendously if you enter grad school builds on 4th year classes so it will help you.
The majors are related ie communications major going to learn the field that career and will demonstrate your commitment to train you and they expect you some experience in marine.
For marine biology then apply again in biology worked as lab technicians after college good luck if you some experience in marine biology then apply again in biology without more relevant major going to have basic level of proficience in biology then apply again in the graduate program is making commitment to have.
An undergrad and ask him this question he would have better idea on whether or not you have already targeted professor to work with why dont you are qualified.