In Fantastic Voyage, the miniaturized submarine is called Proteus. The vessel carries a five-person mission into the body of the comatose scientist Dr. Jan Benes, whose knowledge of indefinite miniaturization may be vital to the world’s governments.
The crew includes intelligence agent Charles Grant, submariner Captain Bill Owens, medical specialist Dr. Michaels, surgeon Dr. Peter Duval, and assistant Cora Peterson. After the submarine and its crew are reduced to roughly the size of a microbe, they are injected into Benes and travel through his circulatory and lymphatic systems toward a blood clot in his brain.
The name Proteus comes from Greek mythology, where Proteus is a sea god associated with shapeshifting. That makes it an apt name for a submarine in a film built around radical changes of scale and form. The movie’s other possible answers are real or familiar submarine names, but they do not belong to this fictional craft: Nautilus is associated with Jules Verne and later real vessels, while Seaview belongs to Voyage to the Bottom of the Sea.