Great find. And you did the right thing, in case you find another; you can simply ask the police to run the serial number. Takes a couple of minutes. If it’s clear, once cleaned up that’s 200 bucks in you pocket. Or a smooth looking Columbo snubby in your truck.
Very nice find. I would have done the same thing, by turning it in. You don't want to be caught with that in your possession. They really wanted to get rid of the evidence by throwing it away in the holster ! Like A2 said, I would check with them and see if you can reclaim it if it wasn't used in a crime. Of course, depending on your state's laws, you may have to apply for a foid (firearm owners identification) card and register the gun in your name after the fact. That may not be worth it to you.
Being that it is belt type holster, kinda makes like an "on purpose" throw away. I have found the same with a clip on holster which may very well indicate an accidental dropping in the water. Letting the authorities deal with it is the best way but have it returned if it is proven that was not part of an investigation. An item like that cleaned up and traded at your local sportings store (the legal way) could result in funds enough to maybe get some good rope, tote bags or other items for the hobby that allowed you to make that great find. Good luck with your choice.