Hunting open water is far different than hunting puddlers in a marsh HOWEVER, the gun you use and your shotshell of choice is no different.

I hunt the Horicon area for puddlers and geese and then hunt Lake Michigan for divers and we get a few Scoters too as well as Old Squaw.

I use a Binelli or a Berreta and use 3" #3 Steel - both guns have a Modified choke for everything and do just fine.

The difference in open water of "Sea Duck" hunting and if youhave a partner we have a rule.

Duck hits the water and head pops up - "Swat" and I mean now! Or you will loose them.
JW