Once you start your A-levels, you'll start looking at Unis. You can go on visits, read their prospectus, speak to course directors and find out which course will suit you. Also, you may find that a biology or similar undergrad degree is the way to go and then do a PhD in Marine science; a PhD is usually the way to go for a career in science as your career is often research based.
Usually Unis that do such a degree are located on the coast, so that might narrow your search. You can look at the UCAS website and they let you search by degree title etc. and it gives some course descriptions.