Here's a photo of a Black-vented Shearwater, taken earlier in the day, for comparison purposes. Besides the dark vent (undertail), also notice that this bird has much more mottled underwing linings, and presents a less "contrasty" overall appearance than the Manx.

The previous Manx Shearwater photos also highlight the semi-circular white flank patches ("golf-ball effect"), the pale arriculars, and blacker upperparts.

I think this was the 6th trip this year on which I've seen a Manx, but it's the first on which I got a photograph of one. As for the Black-venteds; well, they just arrived in Monterey Bay one week earlier after having been absent all fall. Aaah, the joy of the changing seasons....