Use dead/shot ducks or birds. If the dog is birdy enough they will hunt until they find it. The scent will help with that too and scenting/tracking is an important part of a complete training regiment.
Beyond that, I used the command "Find It" when my dog goes into hunt mode. If it is a mark in thick cover then I goad him with the "Find It" command. If it is a blind HUNTING retrieve and I get him in the immediate area, I use "Find It" to let him know that I've gotten him as close as I can get him and he needs to take over at that point. I also use "Find It" to get him to start searching under other circumstances. Several times we've had ducks swim off into tall grass around our hole during a hunt, usually when there were other birds still working or other dead birds out. One time we knew a drake was down in the tall grass and we just waited until after the hunt and I took my dog over there, commanded "Find It", and he knew to go into sniffing mode. It was two hours after the duck was shot.
"I'll start spending less time with my dog and more with my wife when she starts fetching ducks for me"