This was a "grenade like" device. A grenade more than likely isn't going to do much damage to a dam, especially underwater.
My guess is someone found, or had, an old grenade laying around and started thinking about the consequences of having it, so they got rid of it by tossing it in the water.
Locally, about 5 or 6 years ago they were doing maintenance on a bridge that runs across the Snake River. While the divers were down there doing their last checks they found an unexploded artillery shell at the base of the bridge. Best guess on this is that somebody found it laying around at one of the old ranges out in the desert and then didn't know what to do with it. So, they threw it off the bridge. It had been down there for years.
I've found, and have hanging on my wall, a belt of unexpended 20mm shells that I found while squirrel hunting. If someone is so inclined, you can still go out to the old gunnery ranges and find unexploded ordinance. Probably not wise, but it's still laying out there.