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Seals
Seals live a dual life, dividing their time between life in the sea and on land. Consequently, they have adapted to both environments. Being less than agile on land leaves them open to predators, so seals have developed a flexible neck to keep a watch out for danger. When in the water, seals transform into maneuverable and adept movers, but the cold, murky ocean water causes problems of its own. Seals have adapted long whiskers to combat the lack of visibility by feeling for fish in the darkness. They have also developed fur to retain warmth and the technique of 'sailing' -- raising one flipper above the surface -- to cool down when they get too warm.
Manatees
Also known as sea cows, these marine mammals inhabit warm, shallow waters like those just off the Florida coast. Although ungainly in appearance, these herbivorous animals have adapted a powerful tail to allow them to accelerate to great speeds underwater. Florida manatees have been known to reach speeds of up to 15 miles per hour over short distances, which is perfect for escaping marine predators. Because they live in shallower seas, manatees have no need to dive to the depths of their mammalian cousins, the whales and the dolphins. Manatees have not adapted the high concentrations of myoglobin in their blood stream that other marine mammals use to retain oxygen. They are able, however, to slow their heartbeat when diving in order to be able to stay down for longer.
Whales
The largest of all the marine mammals, whales have developed some fascinating adaptations to survive in their environment. One of the most obvious is their blowhole, a respiratory opening positioned on the top of their heads. This negates the need for the animal to haul its massive bulk out of the water to breathe. Whales cannot breathe while unconscious, so they have adapted to be able to sleep with half of their brain at a time, while the other half remains conscious to regulate breathing. Unlike manatees, deep-diving whales have developed large levels of myoglobin in their blood, to retain oxygen while diving deep beneath the waves. This is especially concentrated around vital organs such as the heart and lungs, keeping these organs supplied with oxygen.
Dolphins
Dolphins have evolved similar adaptations to whales, including a blowhole, myoglobin in their blood and blubber to keep warm. Because of their smaller size, dolphins are more vulnerable to attacks from ocean-going predators, such as sharks. To combat this they have developed a strong, pointy nose to ram marauding sharks with, as well as an enormous brain relative to their size to aid evasion. Dolphins also only have one set of small teeth, perfectly adapted to catching their main prey: fish.