While scouting is still important when hunting over water, it seems the locations that geese will use are much more predictable on water than on land. Sandy shorelines, the lee sides of islands, shallow sandbars, shallow bays or points are places to set up for geese. Geese quickly learn that shorelines mean danger, so the best locations to set up on are near islands, off points or by using a boat blind to get away from the shoreline. Getting away from shore also makes your decoy spread more visible. You can tuck a few decoys backing the cattails, but keep the bulk of your spread out in the open where they're highly visible.
That doesn't mean that you can't do well with a shoreline setup -- it just takes a little more convincing. If possible, I like to employ a Paul Revere spread -- one if by land, two if by sea -- when hunting shorelines. I combine my floating goose spread with some field silhouettes or shells. The floaters are placed in shallow water just offshore, and the shells and silhouettes are placed on a sandbar or along the shoreline. The decoys in the water simulate geese that have just landed and are joining the group. The decoys on shore look like birds that are content, preening and resting. The varied positions of the silhouettes give added realism to the spread. In fact, if there isn't a sandbar to put field decoys on, I get double duty out of my field decoys by cutting 2- to 3-foot lengths of 1-inch PVC pipe, spray paint them black, push them down into the shallow water and then drop the stakes of the field decoys into the pipe. Silhouettes in particular can give added realism to your spread, they take up little space and they can pump up the number of decoys in your spread.
The beauty of hunting geese on the water is that you don't need as many decoys as you do in the field. Usually three or four dozen on water is plenty, but it helps if they look ultra-real.
Many floating goose decoys have only sentry or resting heads. Geese with their heads up are alert, if not alarmed. You want to give the impression of contented, resting birds. To do this, add some sleeper and feeder decoys to your spread. Goose butts or feeders are highly visible and give the impression that there might be some aquatic vegetation in the area worth checking out. Limit the number of decoys with sentry heads in your water spread to just a few. Buy the best decoys you can afford. The added detail of quality decoys will help fool educated geese. Cheap decoys with rounded bottoms or water keels tend to rock unnaturally in even the slightest waves.
I generally set the decoys in a wide C- or J-shape with the open end facing downwind. I'll place one decoy at each end of the spread and at the outside of shooting range, or about 40 yards, and then fill in the blanks. Ideally, the wind should be at your back, but there are times when geese are spooky that a crossing setup works better. Place the bulk of the decoys on the upwind side of the spread. Decoy spreads can be bunched a little more tightly when hunting water versus on land. Put your feeders and sleepers on the inside of the spread, and the sentry decoys at the head and tail of the spread. I like to leave an opening directly in front of the blind and then put a half-dozen decoy slightly downwind of the blind. The small group simulates birds that have just landed and are swimming into the decoys to join in on the fun and investigate. When using flying decoys, I place them at the tail of the spread to mimic birds that are just coming in to land. Ideally the geese will be landing head-on when you take them to expose their vitals and ensure clean kills.