I found this a good definition of a Marine Biologist. But, it is one of many…
http://www.wisegeek.com/what-is-a-marine-biologist.htm
A marine biologist is usually a person with advanced degrees in life sciences. He or she will study life forms of the ocean from a scientific perspective, and may hold specific bachelors, masters, or PhDs in biology, marine biology, and/or chemistry.
There are many different areas in which a marine biologist may work. Very few marine biologists work studying ocean dwelling mammals like whales or dolphins. Yet people often think this is the only area in which a marine biologist can work.