Elephant seals amaze scientists with their ability to sleep 1,200 feet deep under the ocean to avoid predators
Soooo...you can file this under "things I never actually thought about before but it makes sense". The short version is that they essentially take underwater power naps that can last up to about ten minutes. They spiral down deeper into the water as they snooze to avoid predators that tend to stay closer to the surface. This lets them stay safe while getting a bit of shuteye.
They do also sleep on land, up to ten hours a day, so it's not that they don't have any other opportunity to sleep. I do wonder if the increase in human activity on the ocean over the past couple of centuries has affected their land to water sleep ratio, though. It'll be neat to see if more research is done on how elephant seals and other marine mammals sleep and whether we're having an impact on that.