I made a few quick 1-minute counts. The average was around 9 individuals passing by a spotting-scope-view per minute. The highest count was 29 individuals/minute (15 Common Loons, 8 Pacific Loons, 6 Red-throated Loons).
I didn't get great photos today, as it was quite windy and the birds were pretty far out. I'll show a couple of images so you can see what the conditions were like and you'll know what to look for...but then I'll add a couple of better shots from last year when the birds were closer to shore.
I'm really not sure it's worth posting today's images, but here goes. The first shows three Pacific Loons. If you look very carefully, you might be able to see their pale gray heads. The second picture shows two Common Loons. Look for their dark black heads.
Now for the nicer images from 2012.
A small group of Pacific Loons close to shore near Mendocino on 9 May 2012. Now you'll be able to see the pale gray on the back of the head.
A larger flock moving past Bodega Head on 6 May 2012.
In California, spring migration of Pacific Loons peaks in mid-late April and tapers off throughout May, so keep an eye to the ocean for these impressive northbound flights.