If they became fully unconscious while asleep, whales, dolphins, and porpoises would drown:
So [whales] have solved the problem with unihemispheric sleep: that is, they shut down only one half of the brain at a time, keeping one-half conscious and breathing.”
Whales have some of the largest brains on the planet. Sperm whales and killer whales in particular have the biggest brain of any living mammal. This means they can actively decide which part of their brains to use at a given time.
Rose said this peculiar way of sleeping can be seen most clearly in captive whales, as they are easier to see. When whales are “sleeping” they can be seen keeping one eye closed while the other remains open.
Robyn White, “How Do Whales Sleep?” at Newsweek (March 4, 2022)
So whales just “solved the problem”? In reality, without any underlying intelligence in nature, their lifestyle just would not have existed.