Roko, you could consider posting the video and photos in the Critter Watchers section of the Northwest Dive Club bulletin board. Several very experienced folks watch that and provide ID information, including a woman whose specialty is worms.
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
Just some random thoughts:
I've seen many polychaete worms in my field but have never seen that type of swimming behavior. I always expect the unexpected though.
Roko, If you had said you were in tropical FW locale I would say the movement closely resembles that of a knifefish.
I then thought of SW species simular to that and came up with gunnels, wrymouths and/or pricklebacks.
Though the book says "rarely seen by divers" for the graveldigger it could be refering to daytime sightings and may not make note of nocturnal or occasional spawning activity.
It could also be a juvenile or larval stage specimen, many of which bear little resemblance to the adult form.
I have no answers. My money currently is on a juvenile gunnel or prickleback. Thanks for the mystery.