Between BU and Northeastern, I’d give the nod to BU. Northeastern has a good marine biology program, but BU has more well-rounded offerings in marine science (i.e. geological, chemical, and physical oceanography as well). I applied to Boston for graduate school but have never looked closely at Northeastern, however, so take that opinion with a pinch of salt. Both are definitely good enough for an undergraduate’s purposes.
Eckerd has an awesome reputation in marine science. In fact, it has produced more PhDs in oceanography – in raw numbers, let alone per capita! – than many much, much larger universities like Florida State and UCLA, and it has produced more Hollings scholars than any other college. Additionally, you don’t have to worry about competing with graduate students like you do at most universities.