How do walruses sleep in the water without drowning?

If people swam a bit more like walruses, lifeguards might be out of a job. Although clumsy on land, these aquatic mammals become pictures of grace underwater. Their four flippers propel them to speeds of up to 22 mph (35 kph), and their well-tuned circulatory systems enable them to spend more than 10 minutes underwater before coming up for air. Their hardy bodies -- male Pacific walruses may weigh as much as 3,748 pounds (1,700 kilograms) -- can even tolerate frigid arctic waters as low as -31 degrees Fahrenheit (-35 degrees Celsius Even though you may not envy the thick layer of blubber that keeps walruses warm, you might find their ability to sleep in the water without drowning incredibly helpful. Wouldn't it be nice simply to stop swimming and gently bob along in the water whenever you got tired instead of having to haul yourself back to dry land?

(You do know, don't you, that the Walrus was Paul?  http://en.wikipedia.org/wiki/I_Am_the_Walrus)

http://animals.howstuffworks.com/mammals/walrus-sleep-without-drowing.htm