ajohn

I don't claim to be an expert on diver ID, but my feeling is that this is a red-throated for the following reasons:

Head and bill look comparatively small and neck fairly slender
Bill appears slightly up-tilted (although this is difficult to judge on a long-range shot!)
Extensive white on the sides of the neck, with only a narrow band of black down the back of the neck
Extensive white on the rear flanks, coming high onto the sides of the body