This probably happens quite often. Last week while checking out some gear in a local bay I was free diving in shallow water. The viz was about 2 feet and while looking at the bottom I saw a small piece of waterlogged wood scooting along the bottom. This seemed a bit strange to me since I've never seen wood swim! I grabbed it and surfaced. When I got to the surface I found out that it was tangled up in some monofilament fishing line. When I started to gather it up I found it was attached to something. One of my friends joked, "waduh ya have, a fish on the other end?". As I pulled on it a bit I replied "yes, as a matter of fact!". Not knowing exactly what was on the other end, I pulled in a 14" corbina.
Didn't feel like having fish that day so I just cut the line as close as I could (swallowed the hook) and watched it swim off. Maybe it will return the favor someday!
-Mark-