I have come upon Leeches in the shallows of lakes, particularly in B.C. Fly fishing literature promotes the use of Leech patterns for stillwater fly fishing.
Typically, you see the Woolly Bugger pattern offered up as the go to Leech pattern, or maybe a brushed out Mohair or Synthetic Fiber Leech pattern. Check Google Images Leech Fly Pattern and you see the range of patterns and attempts at innovation toward the real critter.
Leeches reportedly move in an undulating locomotion. Tying and fishing a Leech pattern with that in mind would focus upon the presentation of the fly; how the materials contribute to a sense of undulation and is the pattern shaped like a Leech. Many ‘Leech’ patterns are misshapen and presented in a horizontal mode with only the slightest undulation, usually imparted from the bead head.
I have taken to fishing a Leech pattern off the bottom in the shoals beneath a slip bobber. This is particularly effective if the water’s surface is rippled enough to impart a slight up and down movement to the bobber/fly. First light, last light and dark skies are good times to fish those shallower areas where the muck and debris suits the Leech. I have often found them on my waders in that first few feet of water as I am getting into my pontoon boat or back to shore preparing to exit the pontoon. The bottom is often muck and woody debris.