Marine habitats are usually in the ocean or some body of water. Zoology sounds like a good reason, but maybe not because it's the study of watching over the animals behavior and instincts.
Botany is not the answer either because it is the study of plants.
Oceanography is probably the best answer because it has the word ocean in it where marine habitiats are, and I'm guessing it's about the branch of science that deals with the physical and biological properties and phenomena of the sea.
