The video that you see below about teaching your dog to come to you with a whistle in water can come in handy. I use 3 consecutive whistle toots to teach the dog to come.
You would overlay this just like you would use a command. Different dog sports use whistles in a variety of ways, so you can use whatever whistle program suits you.
A whistle also works great when the dog is out of distance to your regular voice. He could easily hear a whistle then.
You do want to start close enough where you could monitor the situation, and then move on to greater distances.
A dog’s natural tendency is just to swim across and get out on the shortest possible route, and the angles have him pay attention more.
It is advisable to have a line on the dog at first if he tries to go straight to the other side and guide him with it. The puppy in this video is only about 3 1/2 months old in the video, but he is already off leash trained with the Sit Means Sit remote dog training collar, so does not need a line.