Many biological and physical oceanographers are teachers and professors. Indeed, there are nearly as many marine scientists employed in the academic world as there are in private industry and government. If you like the idea of teaching, you might consider this avenue. The demand for science teachers at all educational levels is already great and is expected to increase. Employers are favorably impressed by experience, espeically work experience related to the duties of the position for which you are applying.
Volunteer work counts.
Good grades are important, especially for positions in governmental agencies. A GPA of 3.0 or higher in all college work increases your chances of employment and should give you a higher starting salary. Don't restrict you acceptable areas of employment. One of the pleasures of being a professor is hearing from them. One of our former students, at the time an employee of the Marine Science Institute at the University of California, Santa Barbara, recently reported his activities as part of a team using the submersible Alvin to investigate plumes of warm water issuing from hydrothermal vents along the southern Juan de Fuca Ridge.