Good luck to you nix, but can I just say that I know someone very well, who's also planning on being a marine biologist. She's in her second year at uni and only just scraped in on her A's and one B, so you need to work very hard to get good grades. You'll certainly need an A Level in biology, as well as one in chemistry or maths. The job CAN mean going out on boat trips, and sometimes to exotic locations, but these are very few and far between. Looking in New Scientist mag, marine biologists often earn far less than what you've stated, but love the research and work that goes with the job.
I hope the girl at your school becomes a teacher. Good ones are sadly lacking, so we need more of them. If she achieves the good grades which're predicted, she can expect to land a good, steady job one day, which'll also bring as much satisfaction as they job you're hoping to get.