Upon closer inspection, these were large flocks of Eared Grebes (Podiceps nigricollis).
To prove it, here's a zoomed in view of a smaller group, with a few individuals that were starting to dive. (Entire flocks often submerge at the same time.)
But returning to the first flock. I'm wondering how many birds were in this flock. It seemed like a large number. What do you think? Do you have an estimate?
If you're up to this challenge, here are two additional pictures that might make it easier to see individual birds — the first showing the left side of the flock, and the second showing the right (note the same piece of kelp floating in front of the flock in both photos).
I haven't actually tried to count yet (although now I may be counting grebes as I go to sleep)...but let me know if you come up with a number! (Warning: My estimate is below these photos.)
ADDENDUM (24 March 2013): If you're wondering and would like to compare, I did a quick estimate and came up with 1055 grebes!