I had a great experience as a post-bacc student in Oregon State University's Fisheries and Wildlife program. My focus was terrestrial ecology, but I had post-bacc friends that focused in marine. One interned at Monterey Bay Aquarium Research Institute, the other is doing monk seal research in Hawaii, and another has a permanent gig as a wildlife biologist in Florida (kind of rare without a master's). Most students spend a term out at Hatfield Marine Science Center on the Oregon coast (located in the same town as NOAA's Marine Operations Center). OSU also has the Marine Mammal Institute (http://mmi.oregonstate.edu/).
Networking is really key - you have to put yourself out there, get to know faculty doing research, go to conferences, and get as much experience as possible. OSU tuition is really reasonable for residents if you can move here, work, and hold off on school for a year. With your background, you probably wouldn't really need to complete the F&W degree - just stay long enough to get the experience and connections and then apply for grad school.
You can check out the post-baccalaureate section here: http://fw.oregonstate.edu/content/how-do-i-apply-undergraduate-program#pbs