A, the octopus body is soft, with mantle and shell degradation, belonging to mollusks, and A is incorrect;

B, carp breathe with gills and swim with fins, which belongs to fish, and B is correct;

C. Whale viviparous lactation belongs to mammals. It is not a fish, and C is incorrect;

D. Squid is a mollusk with soft body, mantle and shell degeneration, and D is incorrect.

Therefore, choose: B.