We have several watering holes due to distance to water during dryer periods. I maintain them by cleaning them out every few years as they fill up with cat tails and willows. I believe they need to be large enough and deep enough that the water remains somewhat fresh, at least not stagnant. Shade never hurts but leaves will fill your hole and tree roots will cause it to leak. On occasion I rework one and seal the bottom with bentonite clay. I believe midges are attracted to warm, stagnant water, I consider our holes healthy if they have frogs and dont stink.
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It helps a lot to build them where the runoff from the food plot will drain into the hole which keeps it full through more of the year and the water more fresh. but remember to create a spillway to manage overflow so it does not cause bad erosion.