I can't see my creature actually swimming around. Could be possible, but it looks like it would much prefer to be crawling around, rather than swimming.
I've actually got a 2nd one now, but this one is proving to be even more difficult to photograph. This one is the same body shape..long, wormlike, with leglike side spikes and a head surrounded by tentacles....and jaws. This one is smaller and almost completely black, with a white stripe right around the end giving the appearance of a head section.
It really sounds like Jim D. has a similar creature. "jaws" is the word for it when he makes his move. It's nasty looking.
I've got some bristle worms also...very different..no head tentacles and no jaws to speak of. Similar habits, although my bristleworms tend to be bolder and just crawl where ever they please.