I can't help until I see it myself, preferably while holding a shotgun during the season.
Kidding aside, your location sounds great if there are ducks around.
A few geese decoys off the island, some mallards at the entrance to the slough, and a few pairs of duck decoys just down stream near your side of the river.
A dozen mallards and half dozen geese decoys ought to do the trick. I call it the triangle. Birds should naturally go for the slack water.
Still be willing to adjust your spread. Even during the day your are hunting if they don't cooperate to your satisfaction.
Hope this helps and good luck