This doesn’t matter. A PhD is required for a career in marine biology, and you can specialize in grad school. At the undergraduate level, the most important thing is to get a solid grounding in math and all of the sciences – which one can definitely get at Dartmouth. Many PhD students in marine biology did their undergrad at landlocked colleges.
You can get exposure to marine science by studying abroad for a semester/year (e.g. Southampton in the UK or James Cook in Australia), doing an REU in oceanography, doing a summer internship at an aquarium, etc.
https://www.nsf.gov/crssprgm/reu/list_result.jsp?unitid=5053
Dartmouth for undergrad and UDub for grad school seems like a winning combo.
I wouldn’t rule out Dartmouth, but I do agree that it’s worth considering some of its peer schools. As you noted, U Miami is hard to beat.
I’m admittedly biased, but there are few places better than Duke for undergraduate studies in marine biology/science, and there’s certainly nowhere better for environmental policy. Stanford and Cornell are superb as well.
https://nicholas.duke.edu/
https://nicholas.duke.edu/marinelab
https://nicholas.duke.edu/marinelab/programs/undergraduate/RachelCarsonScholars
https://nicholas.duke.edu/career/for-students/stanback
https://biology.duke.edu/undergrad/requirements/concentrations/marine.html
There are several liberal arts colleges with great track records in marine science, including Bowdoin, Amherst, Eckerd, and the New College of Florida. The New College of Florida guarantees a $15K per year scholarship for out-of-state students, knocking the cost down to about $25K a year.