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.