I like the Caddy theory, but never heard of it before.

There were 4 of us in the 1960's era wooden boat with twin 35hp Johnson on the back. I think the boat was 18ft. Whatever my Uncle hooked was towing us around PNP and up Admiralty Inlet when Dad and my uncle decided to cut the line. I think my uncle could have reeled in a shark, and it might have had more action than just towing us. They really thought it was a large skate or a submarine.