Damn, I wish I could tell you what I use...but it is natural.

I think that I have a decent amount of sago or some other form of pondweed. My diving ducks like the ringnecks seem to be feeding on that when I shoot them. Some of it looks like bladderwort, which from reading looks like it is enjoyed by mergansers...and I get a lot of mergansers, so that may be a correct identification. I also have a lot of lillys. I get a ton of wood ducks in the creeks below the house...but I never see them in my stock ponds.
I do all of my duck hunting, pond hopping them after I put out hay for the morning. They don't seem to mind me driving by at 50-100 yards in a tractor.
Best of luck man.