I’m not too sure how the process works with scientists but I can give a rough outline. If you know that marine biology is where your heart lies, then yes, go for it! If you’re not too sure but are keen in the biology side of science, then do biology. Throughout university you will study different topics and in your final year you can choose a topic to specialise in… You might still choose marine biology, or you may have found something you like even more whilst studying! I know somebody who went to university to study marine biology and she absolutely loves it!!!
I know it’s difficult but the only person who can make the decision is yourself! 🙂