You don't just study fish...
You study life in the marine environment.
Whether that is protists, bacteria, invertebrates or fish or whales...
Or whether it is whatever factor in the marine environment (water, geology, physics, chemistry) that impacts that life.
It is a very diverse field that encompasses physics, chemistry, geology, ecology, biology and a whole lot of other fields.
A marine biologist may study coral reefs, rocky shores, sandy beaches, maritime forests, the deep sea, hydrothermal vents, hypersaline ecosystems....tropical to arctic....and lots of other stuff too.