The creature likely derives from a species of dolphin that mutated after the Great War. Nearly all specimens bear scars, suggesting competitiveness among their own species or others, or that they are subject to predation.
This creature seems to be a mutated aquatic species, its appearance suggesting it is related to the order Cetacea, which includes marine mammals such as dolphins, porpoises and whales. It possesses similar physical attributes to dolphins, with gray-white coloration, a prominent snout, a dorsal fin, and a blowhole above the eyes. Unlike a dolphin however, the sea creature has large triangular shark-like teeth, instead of more numerous and smaller teeth.
Intact specimens can be shot to cause the stomach to explode in a bloody fashion. Many dead sea creatures are localized in mirelurk territories and often found being fed on by them. Others appear to have been hauled onto boats by wastelanders.
Only carcasses of the creature can be found beached around the wasteland on boats, rivers and the coast. Another can be found deep underwater to the northeast of Fort Strong, although at such a depth that most player characters could drown or be lethally irradiated before finding it without the Aquaboy perk. The only instance of a living one is in the quest animation for the cut quest 20 Leagues Under the Sea.
The sea creature found on the boat west of Beantown Brewery is a reference to the film Jaws. On the boat, a skeleton with a blue jacket, blue bandana, and a machete can be found with a dead sea creature nearby. The outfit clothing and machete are very similar to the ones worn and used by Quint in the film, while the positioning of the sea creature matches that of where Jaws emerges and kills Quint. Underneath the boat one will also find a diving cage resting on the river bed.