Whales drown if they remain underwater for over 30 minutes.
Well, death by drowning is typically caused by suffocation. Water fills the lungs of the whale, thus preventing the entry of oxygen. Without oxygen in their lungs, they ultimately die from a lack of air.
To avoid drowning during sleep, it is crucial that marine mammals retain control of their blowhole.
Besides drowning, whales can suffocate when they get stranded.