I've caught fish down to 4"-6" of visibility, the correct term is probably stained. I refer to it as mud because I like to fish the "mud line" and people are quicker to pick up on that. Two years ago around the horn we caught over 50 walleye trolling the gravel in those conditions. I think success is highly dependent on where the fish are congregating. But once you find them you can hammer them.
Fishfinder makes a huge difference, no one should be afraid to fish some dirty water is my point.
Edit: here was my report of that exact trip
Bullfrog to the Horn 5/15 - 5/16