A marine biologist was a biologist who studied the field of science related to marine biology, specifically involved in the study of marine organisms. A marine biologist who specifically studied whales and related species was known as a cetacean biologist, or more specifically, a whale biologist.
Gillian Taylor was a cetacean biologist who worked with the humpback whales, George and Gracie, at Sausalito, California's Cetacean Institute in 1986. During a pizza date with James T. Kirk, Kirk inquired to Taylor how "a nice girl like you got to be a cetacean biologist," she responded that she was "just lucky, I guess." Later, when Kirk and the crew of the HMS Bounty attempted to capture George and Gracie to take back to the 23rd century, Taylor convinced Kirk to take her with him to the future because she was a whale biologist, and "who in the 23rd century knows anything about humpback whales?"
Despite this, she was assigned to a science vessel upon her arrival in the 23rd century, telling Kirk that she had "three hundred years of catch-up learning to do." (Star Trek IV: The Voyage Home)
Amanda Rogers' adoptive parents were Starfleet marine biologists, stationed in the Bilaren system as of 2369. (TNG: "True Q")