A marine biologist is someone who works in some branch of marine biology. Given that around 70 percent of the Earth's surface is covered in oceans, you can imagine that marine biology is a very large field, encompassing everything from studies of ocean currents to analysis of the creatures who live next to hydrothermal vents. As a general rule, a person in this position is passionate about and interested in some aspect of the ocean, and he or she usually works to study and preserve ocean life while also educating people about it.
Marine biology itself is usually defined as a study of the marine ecosystem, which includes the world's oceans and some brackish regions such as estuaries. The animal and plant life in the ocean is incredibly diverse, ranging from single celled organisms like diatoms to massive blue whales. Because marine biology is such a varied field of study, a marine biologist typically picks a single aspect to focus on. For example, he or she might specialize in fisheries conservation, or rehabilitation of injured marine mammals.
One thing marine biologists don't do is handle marine mammals. Zoologists more typically work with whales and dolphins, and people who are interested in careers with these animals should pursue zoology, rather than marine biology. But a marine biologist studies lots of other interesting things, like the potential for pharmaceutical products derived from the ocean, or ways in which human activities impact the ocean.
Pay for a marine biologist can sometimes be less than ideal, but many people in the profession feel that the flexibility of the field makes up for this shortcoming. Many people in this field get to travel to interesting places and work with cutting edge equipment, sometimes with a high level of autonomy along with staff assistants. Others may focus on studies in labs, with the hope of learning more about specific ocean organisms and their interactions with one another, while some work in the field of education.
The possible careers for a marine biologist are myriad, and are suitable for people with a wide range of abilities and interests, as long as they are passionate and committed. If you are interested in becoming a marine biologist, you should focus on science as early in your education as possible, and you should try to go to college at an institution with marine biology research facilities. You might also consider pursuing internships and other opportunities for experience in the field, to see if marine biology is the right career for you.