I’m primarily familiar with marine science. The following would benefit your application:
[ul][]Basic courses in biology (intro bio, ecology & evolution, genetics, molecular bio) and advanced courses in organismal biology
[]Intro chem and organic chem (2 semesters of each)
[]Calculus (2 semesters); a background in diff eqs is helpful for work in physical oceanography
[]Physics (2 semesters)
[]Stats (1 semester)
[]At least a couple years of research experience. Look for summer programs at marine labs and the REU programs such as SURFO at URI.
http://www.nsf.gov/crssprgm/reu/list_result.jsp?unitid=5053
[li]Good fit with at least two faculty members at the university you’re applying to [/ul][/li]A course in geography and/or GIS would also be helpful. If you’re more interested in marine policy, you should take an intro econ course (microecon especially).
The “best” programs are the ones that fit your interests…someone interested in marine biomedicine would have a very different list from someone wanting to study cephalopod cognition and behavior, for instance. You don’t need to be thinking about specific grad schools for a couple of years yet.