A lot of the time we get into the habit of always throwing retrieves onto the water for our dogs. This type of exercise is to help work on your dog marking retrieves well and also taking the shortest, most direct route to the retrieve.
This is an exercise you may find in a working test. The dog has to mark the dummy thrown on the other side of the pond, swim over the pond, pick the dummy and then return the same way it went.
This is a challenging exercise as most dogs will be tempted to try and find a way around the pond so consider this when trying out this exercise and try to use areas where it is easier for the dog to return across the water rather than going round.
Practice getting your delivery out of water where the dog brings the dummy into you rather than stopping to shake and dropping the dummy.
Start up-close to the water first and gradually build up distance as your delivery becomes more and more reliable.
You can teach your dog to shake on command after taking the retrieve from them by saying the word shake as they do it naturally, over time they then pattern this as a commanded behavior which you and request when required.