On the deck I was struck by the clear white, grey and black plumage tones. The wing-bars were strong and clear white, standing out very obviously. The head pattern was difficult to see in detail at such a distance, however the Supercilium stood out very prominently and the lores gave an impression of lightness but it was difficult to be precise and the pale 'wraparound' to the ear coverts could not be seen - it was just too far away though can it be seen on a photo?
The plumage and call clearly rule out 'Alba' and 'normal' Yellow Wagtails. However the problem of eastern Yellow Wagtails which can apparently, in extreme cases, lack yellow tones and have a similar call to Citrine is more of a challenge. I would expect these birds to have a less distinct head pattern though, including a narrower supercilium, not showing such overall contrast as these birds.
In context, I would think that the number of Citrine Wagtails in the country recently would be an argument in favour of Citrine, the likelihood of two striking Eastern Yellows together, on the west coast, on such an early date must be pretty distant I would imagine. Eastern Yellows are usually late-autumn migrants. The pictures below are terrible I know but they give a little feel for the birds.