I wish that I would have known about this tip when my lab was a puppy. He is a fantastic retriever both on land and in the water, but in the water Charlie likes to take the fastest way back to shore and then run the bank back to me. This is a bad habit for a retriever and it would not bode well during any competitions.
A method I recently read about for training retriever pups is to go to a small pond that is encircled by tall grass or cattails. Beat down a small path to the waters edge and do your water retrieving training from there. The pup will have no choice but to return to you if they want out of the water. When the pup does get back to you, lay on the praise. After a few sessions of this training it should be engrained into the pups mind that he needs to retrieve directly to you if he wants the praise and the chance to retrieve the dummy again.