The No.4 mag jammed in there and the Philips head band screw suggest to me that it has been in civilian hands. But no doubt an interesting story behind how it ended up as a submersible. Probably an interesting story of how it was found too.
But rifles are sometimes found at creek crossings and other challenging terrain areas on snowmobile and ATV trails. If it falls off in the snow, you are pretty much screwed on finding it until spring.
I have a lever action Winchester (I think it is a Winchester) that I literally tripped over in the bush, just the muzzle and a piece of the barrel sticking out of the ground like a booby trap in amongst the leaves. The mag tube and woodwork had dissolved. I have it in the garage somewhere. It has come in handy for use as a pry bar and as a jack handle.