Oohhh, woe is me ..... there is duckweed in my hole.....oh no..... what will we ever do...... we can't kill no ducks with duckweed in the hole..... the dekes don't move as good as they should.... just STAY HOME!!!!
That duckweed only gets bad enough to negatively affect hunting during years when we have no good summer drawdown --- it spreads primarily during the months of May and June --- and during those months it will spread until it covers the entire surface of the water --- if you get a good summer drawdown, then it will not cover that much surface, and when the water comes from the fall rains to bring the water levels back up, you still have some duckweed, but not enough to cover all of the water ----- under those conditions, you can kill the schit out of ducks that like that duckweed. And yes, gadwalls, widgeon, and wood ducks seem to like this stuff more than the mallards, although the mallards will flock to it from time to time.
Now in rare years when we have a LOT of rainfall in May and June ---during which time water levels are high and this stuff keeps spreading to cover the entire surface, then your hunting could be affected a bit as the stuff may be too thick later during the season. The only solution is to try to kill it with chemicals during the months of May and June to stop it from spreading. If you try to kill it later than that, the chemicals will work, but instead of green duckweed covering the surface, you will have a bunch of brown, dead duckweed floating on the surface which is worse as the ducks won't eat it.
Chemicals in adjacent ag fields that runoff into the water body --particularly phosphates --- cause this stuff to grow even more than normal. There are chemicals that you can buy ---not cheap ---that can kill and help control it ---some you spray on the surface right onto the duckweed --- and some you mix with the water.
So, you can listen to a guy who successfully hunts such holes, or not. The choice is yours. I would not give up on such a hole because it has some duckweed in it. Like I said, it can often be a good thing ---more often than not.