OK... I am not a great dog trainer by any means so please take my advice for what it's worth. Just some stranger on the internet...
I would not advise dragging the dog out into deep water. He will freak out and definitely not learn what you want it to learn. He will learn that you are a bad guy and he shouldn't trust you.
There are two things I would try if it were my dog. First, you could put on your chest waders and wade out into swimming water for the dog. Bring a retrieving dummy and try to coax the dog out to you. Keep tossing the dummy just at the edge of swimming water for the dog. Keep teasing and tantalizing the dog. Eventually he should swim just a little bit to grab the dummy and will then immediately turn around and get back to where he can touch. Keep praising him and encouraging him but never force him or give him any discipline. Keep it all happy and fun and don't try to make him into a water dog all in one outing. Quit while you're ahead and while he is still having fun. Repeat the lesson as many times as it takes until you can throw the dummy from the bank and he will swim to go get it. The reason for the waders is to show him that, "if the boss is out here, it must be safe."