Humans only use one fifth the oxygen in air that is inspired. That is eighty percent of the oxygen is exhaled. This limits our breath hold capability. Whales and dolphins slow down their heartbeat when they dive deep. This is known as bradycardia.
Humans can do it to a much lesser degree.
The rib cage is collapsible. Most humans would develop lung damage due to the enormous compression of lungs under pressure. Cetaceans do not have such a definitive pectoral girdle and the lungs are able to handle pressure changes more efficiently. There are many other adaptations of cetaceans. Types of Cetaceans.