….. Well that’s a rather big question… it can be the study of the biology and ecology of any one of the marine animals and their habitats. With Earth being >70% water, it’s a pretty wide field.
You will, in addition to biology/ecology, probably study statistics, biochemistry, oceanography and more.
It is in no way a beach bum degree. It’s science, and it’s not always easy, but, in my opinion, it is always worth it!