Here's the question I ask myself...If I had a love one killed by a gun and nobody was ever caught, or say they had a suspect but needed more evidence, would I want Joe Public deciding what guns they find metal detecting are not worth turning over to authorities? Of course not, I'd want every person finding every gun to not think for themselves about what they deem worthy of turning in just in case there is one slight little chance they can finally bring somebody to justice.
What if the gun was only recently (last decade or so) pitched in the water and somebody saw the suspect throwing something in the water or he was caught on film doing so. What if they had that kind of evidence and searched the water for a specific gun they are pretty sure he had stolen when robbing a house, and in the act had used to to kill the home owner? Maybe they had their suspect and just needed a bit more evidence to build a stronger case. Maybe that gun was documented with detailed pictures for home insurance reasons and had some distinguishing details that can be seen in pictures?
They may have searched that water right there and didn't come up with what they knew they were looking for, but when a guy metal detecting says "this is where I found it", something clicks and they pull out an old case where their suspect was witnessed to have thrown something in the water at that very spot? If they had that piece of evidence it might not convict the guy but little things like that can add up in building a case. Often people aren't convicted on one solid piece of evidence, but rather a bunch of little pieces that all point to the same conclusion.
You might consider the above to be a ton of "maybes" and a huge stretch, but I'm simply saying that for me that kind of doubt rattling around in my head would kill any enjoyment I got out of keeping a gun, and that I don't feel qualified to decide what I don't need to bother with submitting to the police. All I'm saying is that if there's any chance then I'd have to give it a shot, just so I didn't have any doubt in my mind. That's just me though. To each his own.