Darwin is a crew member of the SeaQuest DSV, the Deep Sea Vehicle that gives its name to the TV series.
In the show, Darwin is a bottlenose dolphin who was found and rescued from distress in his youth by the SeqQuest's Captain Bridger. Some “advanced experimentation” later on gives Darwin the ability of human speech, with a vocabulary of about 125 words both understood and spoken. This, along with his high native intelligence, qualifies him to participate in the SeaQuest's missions. He is trained for deep-sea tracking and data retrieval. A modified torpedo port allows him to enter and exit the SeaQuest at will, and a system of aquatic tubes gives him the run of part of the ship.
Interestingly enough, Darwin is not played by a real living dolphin. Instead, he is a mock-up created by Edge Innovations, a special effects company specializing in animatronics which has also provided special effects for other movies, and series, including Flipper, Free Willy and Star Trek IV.