When the dog seems comfortable pulling the empty jug, gradually add sand, dirt, or water to the jug to make it heavier. By the time your dog pulls a filled jug wherever you tell him to, you are ready for the boat. Train this exercise in water which is belly deep for your dog. This means you cannot see daylight under any part of the dog between the legs. This depth gives them enough traction so they do not have to swim, and is far enough out that most boats will not be close to touching bottom.
Some dogs can tow at an amazing rate, so be sure to stay out of the way of the dog.
Some handlers may prefer to have the dog in heel position and walk beside the dog. Either way, part of the handler's job is to make sure there are no stumps or rocks which could hang up the boat or injure the dog along your path. I always make sure to give the line to the dog so the line goes out of the dogs mouth on the side away from shore. This is important because the boat tends to go towards the side of the dog with the line and it is much easier to pun the boat in from water which is too deep than to pull it off the shore if it beaches. The dog and handler approach the water.