Despite the fact that they are now living in the ocean consistently, dolphins are actually mammals, not fish.
Just like any mammal, dolphins are warm blooded. Compared to fish, who inhale through gills, dolphins inhale air by using lungs. Dolphins should render frequent trips to the body of the water in order to catch a breath. The blowhole over a dolphin’s head works like a “nose,” that makes it easier for a dolphin to surface and breathe air.
Some other characteristics of dolphins which make them mammals instead of fish are that they definitely give birth to a live young rather than to lay eggs and they even feed their young with their own milk. Furthermore, similar to all mammals, dolphins also have a little amount of hair, just about the blowhole, that is a bit different compared to those scales of fish.
Whales and porpoises are likewise mammals. There exist 75 kinds of dolphins, whales, as well as porpoises inhabiting the ocean. They are in fact the only mammals, besides manatees, that devote their whole lives in the water.