Mammals can’t stay under water without holding their breath, like fishes can. But dolphins can stay longer under water than human can, because they take more oxygen out of the air than people do.
A dolphin drowns if he cannot come above the water to take a breath, just like people do. If he gets entangled in a fishnet he will drown, because he cannot go up.
Dolphins are small whales, but the name “whale” is illusive. Dolphins are no fishes: they are mammals. They give birth to living babies and they do not lay eggs. They also feed their babies with milk.
Dolphins have a body temperature of about