No, unlike fish and some amphibious animals dolphins cannot breathe underwater.
This reason for this is that dolphins are marine mammals which means they must come to the surface of the water in order to breathe in oxygen otherwise they would drown; fish and amphibious creatures on the other hand are equipped with gills or specialized organs that allow them to extract oxygen directly from the water.
What separates dolphins from many other mammals is the fact that dolphins breathe through their blowhole, which is located on the top of its head.
Unlike humans dolphins cannot breathe through their mouth because their lungs (air passage) and esophagus (food passage) are separate from each other.
This serves several functions.
First by having a separate hole that is dedicated to breathing dolphins can hunt for prey such as fish while underwater and can eat their food without a fear of inhaling water into their lungs.
Second because the blowhole is located on the top of the dolphins head it’s much easier for the dolphin to breathe while keeping part of its body submerged underwater.
This is especially useful during periods of rest where a dolphins mouth may be submerged underwater, because its blowhole can still be above the surface making breathing easy.
When diving underwater the muscles surrounding the dolphins blowhole automatically contract allowing the dolphin to swim underwater without the risk of inhaling water.
One misconception that is often made about dolphins when they surface is that the water coming from their blowhole is water that the dolphin engulfed.
What actually happens when you see water spout from a dolphins blowhole is the surrounding water being forced into the air when the dolphin is exhaling.
It’s just surface water and not water that has been inhaled because if a dolphin inhaled water it would get into the dolphins lungs and cause damage, and in the worst case scenario the dolphin could drown.
Another interesting fact to point out is that because of the fact that dolphins live in water they never fall completely asleep.
In fact dolphins, whales and porpoises are known as conscious breathers which means they are always conscious or at least semi conscious of their breathing because if they fall completely asleep and lose consciousness they could end up sinking and drown.
To confirm this theory scientists and marine biologists have run tests by hooking up an electroencephalograph (EEG) to a dolphins head and measuring the dolphins brain activity when it rests.
What was captured from the results shows that when a dolphin does go into a semi restful state half of its brain does in fact shut down just like a humans, however the other half of the brain remains active and alert allowing the dolphin to always remain at least semi conscious of its environment in case a threat approaches or if it needs to surface for oxygen.
While dolphins can rest anywhere they are usually spotted resting near or at the surface of the ocean so that they can easily inhale air when it is needed.