For emerger fly fishing, or for any other trout fishing, there are four items always in my fly vest, and I’m surprised when I meet other anglers who don’t carry them.
Shake & Flote: I am hooked on this stuff and cannot believe anyone can fly fish without it. Shake & Flote can be used to pre-treat a dry fly just like any other floatant, and it works as well as anything else for this purpose. Yet it does what no other potion does—it sucks the moisture right out of a drowned or slimed fly, then re-coats the fly with a layer of hydrophilic dry silicone. Shake & Flote won’t mat the hackles of a fly like pastes, and it is the only floatant you should use on a fly made with CDC. It is also the perfect stuff to float and re-float yarn strike indicators.
A knife with great scissors: You can turn a regular dry fly into an emerger or spinner with a few snips. Your Fly Fisherman’s Snips won’t do a very good job; you really need a sharp pair of scissors. This fisherman's multi-tool has a great pair of scissors, screwdrivers for fixing fly reels, a fine file for sharpening bigger nymphs and streamers (you still need a hone for smaller dries), and a blade that takes a nice edge. It’s all nicely packaged in a thin stainless steel tool that opens with one hand and fits in a pocket so well I use it as my everyday pocket knife.
Strike Putty: This stuff goes on your leader and off in seconds. You can use just a smear on a leader to track an emerger’s progress, or use a hunk the size of a marble as a strike indicator. I use yarn for serious deep nymphing, but for shallow nymphing, small stream nymphing, and fly fishing emergers you’ll want to have a tub of this stuff.
Rugged Stream Thermometer: The benefits of having a stream thermometer are many—predicting hatches, finding warmer water in early season and cooler water in midsummer. But have you ever tried to find a good one? Once, on a fishing trip far from home, I lost my thermometer and had to visit several non-Orvis fly shops to replace it. The only ones I could find were hard-to-read digital thermometers or dial ones that look like tiny meat thermometers. They were cheap and inaccurate. The Orvis Rugged Stream Thermometer is a precision alcohol-based thermometer like the ones used by professional fish biologists. As far as I know, Orvis is the only fly-fishing outfitter that sells this quality. Don’t leave on a fly fishing trip without one of these.