I’ve proved it to myself that it is possible to train a dog to retrieve from under the water and pick ducks that keep diving when they approach.
Most dogs watch for the duck to come back to the surface and then take hold of it.
With experience and plenty of practice they often learn to put their heads underwater whenever a duck dives.
Some actually go right under and disappear from sight!
I did once teach a dog to do just that by starting with a golf ball in a small amount of water and gradually increasing the depth so that it first put its nose below water level, then its eyes, then head and, finally, body.
Best of all was a pet Cocker, retired from work, that I taught to retrieve golf balls from the bottom of a large water tub of water – she became obsessed with retrieving underwater and would dive down even when she knew there was nothing to retrieve.