Mammals are a special group of animals with a combination of characteristics that separate them from all others: mammals are warm-blooded, have hair or fur, breathe air through lungs, bear live young, and nurse their young with milk produced by mammary glands.
Marine mammals have the same characteristics as all other mammals, but have adapted to living all or part of their life in the ocean. To keep warm in the ocean, most of them depend on a thick layer of blubber (or fat – like many humans!). They have streamlined bodies to help them swim faster. To be able to stay under water for long periods, they store extra oxygen in their muscles and blood but they do have to surface occasionally unlike their fishy friends. They also have more blood than land mammals in proportion to their body sizes, can direct their blood flow to only their vital organs (such as their heart and lungs), and can slow their heartbeat down so they are using less oxygen in a dive.
For those looking to swim or dive with some of the largest creatures in the oceans, there are some 120 varieties of ocean-dwelling mammal or ones that depend on the ocean for food with five main groups: pinnipeds (seals, sea lions, fur seals, and walruses), cetaceans (whales, dolphins, and porpoises), sea otters, sirenians (dugongs and manatees), and polar bears. Probably of most interest to divers and non-divers alike are the cetacean species and encounters with these in our oceans are experiences never to be forgotten.