Beluga whales often swim in shallow waters at depths that barely cover their bodies. In general they are not thought of as deep diving marine mammals, but they are capable of making deep dives and staying under water for as long as 15 minutes. Why are they able to do that?
Like other marine mammals, beluga whales possess a number of physiological adaptations that we humans do not have that allow them to dive to deep depths and stay under water for longer periods of time than we can.
During dives beluga whales have a slower heart rate. Also, while diving, blood is shunted away from tissues in their bodies that can tolerate low levels of oxygen towards the heart, lungs and brain where more oxygen is needed. Beluga whales possess more oxygen in their blood than most other animals. And, last, but not least, the muscle of belugas has a high content of an oxygen-binding protein called myoglobin. This protein stores oxygen and prevents muscle oxygen deficiency.
All of these adaptations enable the beluga whale to conserve oxygen while under water and do something we humans can only do through artificial means.