Steller's Sea Lions
If you look closely at this picture, the sea lions seem to be more or less laying on top of one another. Occasionally a flipper would go up into the air as seen here and sometimes one would lift up its head to look around, but overall, they just seemed to be using each other as head rests. Maybe this makes it easier for them to keep their noses above water while sleeping.
Location: John Brown's Beach near Sitka, Alaska