Sounds like you just gotta work more with the doggie PFD. Buster (our 2 year old Golden) didn't like his vest when he first got it and used to try to chew through the straps and fabric. Now he associates it with going out on the boat and he doesn't pay any attention to it. Just took a few times with it.
At least that way it improves his visibility in the water so if he does swim to the point of exhaustion people would be less likely to hit him, and you would be able to spend less time babysitting the beloved pup
