Did they resemble bristle worms at all?..... any part of them? During repro BWs change into two parts..... well one part changes and the other stays the same. The atoke which is the front end (unchanged part) and the epitoke which is the back portion. Then when the time is right, the epitoke separates itself and swims to the water surface. Usually there is one that one worm participating much like you describe. They swim in a cork-screw type fashion. I have seen it several times.

I'm not sure if this is what you are seeing or not but I thought I would throw it out there.