I just know, in all my reading and experiences, I don’t think I have ever heard or seen anything about not being allowed to shoot across a river or navigable waterway.
If so, I have literally broke the law in Idaho countless times, in front of both USFW and IDFG officers, I’m talking shooting bears, waterfowl, deer and elk, and never once have I been so much as warned or even heard this come up.
From Hells Canyon, to the Lochsaw, Salmon, upper Snake, Henry’s Fork, Bear River and everywhere in between.
I have had IDFG wardens literally watch me shoot bears across a river, even helped me get my raft launched and picked back up while I retrieved the bear.
I have been checked by Oregon game wardens in hells canyon, with my boat sitting on the Oregon banks, glassing across the river for bears in Idaho, never once has anything come up about this.
I have had USFW officers, (these guys were some of the biggest dicks in the world) search every square inch of my waterfowl boat looking for anything, I mean 200 questions, ripping apart my boat, search every crack and crevice, you name it… This was after they watched us shoot across a river for 3 hours over decoys hunting ducks and geese…. They said the guns appeared to change in audio pitch between shots. (They thought we were mixing in and shooting lead)
Long story short, if these guys could have found something to charge us with, they would have. Never once did shooting across the river other boats were coming up and down even get mentioned.