After posting from my mobile phone, I took a good long time to study this bird. Still calling it a Pacific Loon. It was different in many characteristics from the Common Loon, though they never came near each other for a good comparison. I could repeat my field notes here, but they pretty much match the Sibley description and drawing of juvenile PALO - except that I couldn't detect a "necklace." National Geographic guide says some juvenile PALO don't have a necklace. Or maybe I was too far way to see it.
Would appreciate confirmation by a more skilled birder (if it hangs around)
:)