Surfing today, I got sucked into a nice little barrel, was going alright until the board got sucked from beneath me and I came down and smacked my arm on the fin. It killed, and now there is a visible dent in my arm and a red line from the fin. Can't flex or pick up anything without pain. Is this just a bad bruise or could I have torn it?

You have most likely given it a good 'corky' mate. I doubt you would tear the muscle from an impact directly on it. May have dont a bit of surface soft tissue damage and if the bruising is bad maybe damaged a few little blood vessels but thats all. Just R.I.C.E (rest, ice, compress, elevate) it. Should help with the pain.