How do mammals breathe underwater?

Mammals cannot breath water. Dolphins, whales, seals, walruses, and other aquatic mammals all have lungs and breath air. They seem as if they are breathing water because of the assumption that they have to breath as often as we do. However, aquatic mammals can actually hold their breath much longer, ranging from around 10min for a walrus to over an hour for some whales.

The reason they can do this has to do with the construction of their lungs and the Diving Reflex, which slows heart rate (among other things).
Mammals, such as whales and dolphins, have to hold their breath while underwater. They all have lungs and breath air and would drown if they tried to breath underwater. However, they can hold their breath for a long time (over an hour for some whales).