I've been big clusters (low hundreds) of these birds floating close offshore in Vancouver.
they're not big, pretty small. About the size of a pigeon.
They clump together in large floating rafts which is why a "solitary loon" pattern does not fit. The clusters stay put for a long time, with little arrivals or departures.
They dive often, but don't stay down long.
Top is black, breast is white. The neck seems black, which looks slight different from the white extending the bottom half of the murrelets' heads. Coloring is uniform, no patterns, no differences between individuals.
Don't look very endangered to me, there's a lot of them
They fly extremely quickly in groups, very close together and very, very, low on the water.
I think I mostly see these groups during winter, which would fit murrelets.
Sorry for the picture, if I walk by with a proper camera, I'll repost, but this was using cellphone under low light, zoomed.
With the updated images, I am not totally sure all of them are of the same species. The ones with yellow bills seem different, even if they are together. Interested in the black-billed ones with white-on-black chevrons near their shoulder.