That's a very interesting sighting, and it's an extremely rare privilege to see a sea otter in Oregon! The Elakha Alliance is a group trying to restore sea otters in Oregon, and they are asking people to report sightings to ODFW.
One possibility is that it could have been a river otter, because they're more common than sea otters on beaches. They're often seen in coastal streams, bays and estuaries, and won't hesitate to swim in the ocean, and they also travel fairly long distances away from fresh water along beaches, either on foot or by swimming in the ocean.
As a kid I saw an otter killing a gull at the end of a jetty, and on a couple of other occasions saw otters running down the beach and into the surf. At the time, I thought they were sea otters, but when I learned that sea otters were pretty much extirpated at the time, I realized they had been river otters. But now that sea otters are starting to show up on the Oregon coast, it can be tough to distinguish the two species. Sea otters occasionally come ashore to rest, but they are large (averaging 50-75 lb) and tend to move awkwardly on land.
Lisa Millbank