I focused on Marine Invertebrate Zoology in grad school, but did a general bio degree as an undergrad- with marine bio courses. IMO, get a general science degree as an undergrad, and be as diverse as possible. There are not a lot of Marine Bio jobs, and it pays to be able to cover a lot of material, and have a broad background (all the core sciences, math/statistics, ecology, genetics, etc). Plus, you may learn that in fact you like another field of science better. I think a general science degree will get you a bit further, esp. if you go somewhere where you can take a few marine bio courses (or summer courses at another unversity).
Do as much undergrad research as possible. Get field experience.