DU magazine had an artcle a few years ago written by Mayo Kellogg teaching your dog to dive. In his system he had a dummy floating in the water. The dummy was attached to a rope that went down to a pulley on the bottom, from the pulley the rope went to the handler. When the dog went to retrieve the dummy the handler could pull the rope which pulled the dummy below the surface. Mayo repeated this many times a little deeper each time until the dog was diving.
I don't remember how the dummy was attached to the rope, but I imagine you would need something where a dog went for the dummy he would receive the reward of retrieving the dummy. Maybe velcro or something that would easily pull away. You might need to add water to the dummy to reduce the bouancy so the dummy doesn't break as you submerge the dummy. This is one idea, but you might have to teach the retrieve.