i have a marine science degree from the states, and let me just give you fair warning about the field. it is great and interesting and all, and almost glorified...until you graduate and then what? there are so few jobs for us marine folk that it is very very difficult to find work in the field. i am now a biology teacher and wait tables to make extra cash...most of my friends have also abandoned the field...it is a very competitive, cut-throat sort of degree...
if you DO choose to persue it, my suggestion is to go STRAIGHT on to even higher degree once youve done your undergrad...seriously! do not wait. also try to have a clear picture of what you would like to DO with a marine science degree...if you want to train dolphins or something like that. there are other fields that will lead you to that goal a lot faster, like psych-bio...its all about animal trai ning... researching ocean stuffs is more the marine bio side of things...so if you wanted to say, study coral bleaching or a specific environmental anomaly, than marine bio is more likely the field.
just give it some thought and have a backup plan for when you cant find work in that field after graduation...sorry to sound so harsh but it is the reality of the field.
good luck