Thanks Mike for this very detailed and helpful analysis of Diver ID. The bird looked very black and white when I first saw it, but I agree this can be just a light effect, especially over the sea.
In these situations, you only have a few seconds to look at the bird (I can estimate very accurately from the timings on my photos, the bird took less than 10 seconds to pass me from the time I first spotted it). You have to make the choice between watching the bird or using the camera, there is no time to do both. I guess if I had used this time to just watch the bird, I may have seen the upturned bill. But now I always choose to use the camera as a more reliable and permanent record.
You can see nothing of the features through the camera viewfinder at this range, so it is a risk. But the resulting photos almost always justify the choice, particularly when other more experienced observers can comment on an image and not just a written word.