Cetaceans (whales and dolphins), seals, sea lions, walrus, dugongs, manatees and sea otters are well adapted to life underwater and are capable of diving to great depths. Yet they, like us, are breath hold divers who face the challenges of drowning due to lack of air, coping with water pressure and decompression sickness.
The fundamentals of marine mammals' adaptation to survival in water are demonstrated to a lesser extent in terrestrial animals in an innate self preservation technique called mammalian diving reflex (actually this reflex also applies to birds and reptiles). If the face of a reptile, bird or mammal is submerged in water colder than 70 degrees F, the receptors in the nasal cavity trigger a response in the brain and nervous system which causes 3 main responses. It slows the heart; it slows then stops blood circulation to the extremities so that the heart brain circuit is preserved, thirdly at depth it causes a blood shift, which allows plasma and water to pass through the thoracic cavity, to protect from the effects of pressure. Consequently, even land dwellers such as humans; can survive longer underwater without oxygen, than they can on land, whether they are conscious or unconscious.
Marine mammal have many more complex adaptations to capture, conserve and store oxygen to facilitate their life below the waves.
Another challenge is decompression sickness or the bends, a condition that occurs when nitrogen gas, compressed in the bloodstream at depth, expands during ascent causing pain, injury and even death.
Observation of a wild 82 kilo sea lion who was tagged for 48 dives (after the study, the gadgets were removed and the animal released) showed how the seal conserved oxygen and coped with pressure and nitrogen.
At a depth of around 225 metres, there was a dramatic plunge in the sea lion's oxygen pressure, signalling that it had collapsed its lung to shut off additional air (and thus nitrogen) to its bloodstream. The sea lion reached a depth of some 300 metres before beginning its ascent. At around 247 metres, the oxygen pressure rose again, pointing to a re-inflation of the lung, and then fell off slightly before the sea lion breached the surface. A reserve of air was kept in the upper airways and this was used to re-inflate the lung during ascent.
Impressive as the California sea lion is for diving skills, it is still outstripped by the emperor penguin, which can reach more than 500 metres, and the elephant seal, which can forage at more than 1500 metres.
Marine mammals are not immune to decompression sickness or injury due to rapid ascent. Recently hundreds of dead dolphins were washed up along the beaches in Peru, fishermen reported thousands dead in the ocean. The cause of this disaster is disputed. Necropsies indicated acoustic trauma and decompression syndrome consistent with a loud noise event possibly seismic testing used in underwater mining exploration.