if you are going to have that kind of depth, I would go with some type of SAV. maybe get Pennsylvania smartweed and/or barnyard around the shallower edges, or dig a sizeable flat on the side opposite the dam for moist soil or planting each spring summer. in the depths, id try to get wigeon grass or southern naiad growing. as said above, duckweed is great if you can get it growing, but can also ruin a pond if youre trying to grow fish also. gadwalls and wigeons love it, but it will also wash out during big rain events.