Sinking birds can and do happen in real hunting, as well as shot fliers in tests. Some ducks when hit lay flat in the water or dive to escape the dogs. You can simulate this by having a sinking bumper.

You need a device to use as an anchor (sand filled milk jug gallon size works good. You want to do this in wading depth water, so you can reset the "sinker" without a boat.

Tie a fishing line to a bumper (sort of heavy weight...20# test or so (the fish line that is). Slide the line through the handle of the milk jug (or boat anchor or cinder block) and let the bumper float.

Next you will need a rock or two (we always have a bunch in our pockets when training or hunting, you never know when you may need them!). Do not do this at a long distance, just 20 feet or so, maybe less to begin with as it is the sinking you are working on, not the distance. Let the bumper float for a bit, then pitch a rock at it while you say "mark" or whatever you que is for "something is happening look!". When your dog has marked the bumper (and rock splash) send her. As she swims (or runs) to the bumper start to sink it by pulling the line threaded through the anchor. At first, just get it beneath the surface. As she gets better with the game you can sink it deeper. When she gets the bumper, let go of the line...that's why you don't want it too far away or too deep so she doesn't have to go too far dragging the fish line as she could get a bit tangled, although that will give her practice swimming tangled with decoy lines. You probably could do this with a dead bird too, although I have not done that, as once the guys get the hang of diving they seem to figure it out for themselves. The dogs seem to get into the game after a few "dives". Some turn into great rock retrievers too!!