The correct answer is option 2, Shark. A shark is not a mammal, but rather a type of fish. Sharks belong to a group of fish called cartilaginous fish, which have skeletons made of cartilage instead of bone. They breathe using gills and lay eggs to reproduce, characteristics that differentiate them from mammals.

On the other hand, options 1, 3, and 4 are all mammals. Dolphins, seals, and whales are marine mammals that possess several key mammalian characteristics. They breathe air, have live births, and nurse their young with milk produced by mammary glands. Additionally, they have warm-blooded metabolisms, meaning they regulate their body temperature internally.

In summary, although all options given are marine animals, the correct answer is option 2, Shark, as it is not a mammal but a type of fish.