Ducks and geese have different family social structures, therefore how you setup decoys for each species CAN be different.
Geese mate for life, and the male and female stay together year round. In the fall geese travel in family units; male, famale and young. They may be joined by other (usually) RELATED familys to form flocks. These flocks generally stay together on resting waters, fly out together, and feed together. The flocks may be joined by unmated birds of their own clan, or by unrelated birds, or even geese of different subspecies and species.
Ducks on the other hand, do not travel in family groups during the fall. Within a couple of months of the time when young ducks learn to fly they often leave the hen. Therefore they do not travel in related family groups in the fall. Pairs of males and females may travel together with unrelated unpaired individuals of either sex in the fall and winter.
Ducks often join geese when they feed in fields. Large flocks of ducks may stay in small groups when they feed with geese, small flocks of ducks may scatter among the geese.
When you are hunting on land you probably don' t need duck decoys if you are using goose decoys, but it can' t hurt.
As to hunting ducks on water, ducks feed more on aquatic vegetation than geese, geese feed primarily on land. Therefore, ducks feed more on the water than geese do. Peak feeding times for both ducks and geese are morning and evening. So, you can hunt ducks on the water either time, just be sure there are ducks feeding/resting where you want to hunt.
If you want to learn more about the biology/behavior of ducks and geese, and how to call and hunt them, log on to the " T.R.' s Tips" forum and join us. If you have questions feel free to ask.
Gos bless and good hunting,
T.R.