When I wrote the Florida river monster blog I mentioned this attack and I thought it may be an urban myth. Always ready to be proved wrong I investigated further and in Tim Dinsdale's book "The Leviathans" (1976 ed Futura London Paperback),I found the following which I have scanned in. You should be able to click on it and make it bigger to read comfortably.
It is a letter Tim Dinsdale received from the surviving diver which tells the story of what happened. What they encountered could have been a large seal of some kind possibly a leopard seal .Leopard seals are aggressive and one pulled a young researcher under the ice a few years ago and she perished. What it would have been doing in that area I have no idea. Some will say the survivor was hallucinating or traumatised because of the loss of his friends and so made the story up, not consciously but as an explanation to ease his pain. I don't know , all I can say is read it for your self and come to your own conclusions. If it is true it is a frightening episode.