I wasn't kidding. It has nothing to do with the type of water you are hunting, be it open water or marsh. It is what type of pattern you are hunting. If birds are coming to where you are sitting. Less is more as far as decoys are concerned. If you are going to sit somewhere along the flight path and hope to convince birds to decoy, more decoys are a benefit. This applies to hunting marsh, lake, river, field, whatever.