For flyfishing, the more visibility there is the better, but it's surprising how little you can get by with. I know it's plenty clear enough to fish if I wade into a stream and still faintly see the toes of my boots in knee- or even calf-deep water. If it's any cloudier than that, I'll lower my expectations, but since the stream is still full of food, the trout are still hungry, and there I am in waders, I'll usually give it a try. My standard rig is a brace of flies: a size 4 or 6 dark stonefly nymph, trailed by something like a size 12 or 14 mayfly or caddis larva. I'll use as much weight on the leader as it takes to get the nymphs to the fish. You don't want to snag bottom on every other cast, but if you don't hang up now and then, you're probably not fishing deep enough. And although I'm not a big fan of strike indicators, I'll probably use one. They're no fun to cast, but I like an indicator large and buoyant enough to act as bobber, suspending the nymphs in the current.