We did this once, in total fly-by-the-seat-of-pants ad hoc fashion. We were planning to hunt high tide, which allowed us to hide on shore with natural cover and a little brush. Birds didn't really fly until well into the falling tide, at which point the mud flat in front of us put landing birds out about 50 yards. My buddy noticed a good clump of wild rice that had a little water and we hunkered down in it, sitting in the rice in water about belly deep on us, with just 2 cork black duck decoys out where the birds were landing.
Worked like a charm to decoy birds. The shooting was another matter. Belly deep water worked fine for cover, but where do you put the gun? VERY uncomfortable to hold the gun high and dry. Nowhere solid to rest it. You can rest it on a shoulder or a crooked elbow, but try making nice gun mount on a decoying bird from that position. Great bird watching, but poor hunting.
I assume those who do this regularly find a way to rest the gun in something like a ready position, maybe on a shelf built into stick-up decoy or a portable grass mat or similar screen for cover.