This is the case of a perfectly good dog "going feral".
I am always amazed by the stories of owners who believe that their dog is part of the family and wouldnt hurt a fly or do anything "wrong". It seems sometimes that people forget that dogs are animals descended from the wolf, and then finally honed into whatever shape and behavior that human breedeers decided that they wanted.
Most people buy dogs from pet stores or from rescue shelters and have no idea what it really takes to satisfy a dog. This means mentally and physically providing what the dog was designed to do so that it can live a happy life, not just live in our houses. Given the chance many dogs have forgotten what they were bred to do and have to be subtlely reminded.
In case you were wondering the dog in this photo is a cocker spaniel cross with a poodle. These breeds are both water dogs and retrievers. In fact the cocker got its name from retrieving WOOD COCK birds for hunters. This is why I am never amazed that my beautifully clean dog will just about always choose water whether it be a swamp, a puddle or the ocean over almost any other activity.
Yes he is domesticated and trained, but his inate preference is to find birds, to retrieve birds and to be in swampy water while doing it. Knowing what your breed of dog specifically needs can go a long way towards making them happy.
Another hint, as all the previous post will atest, is to provide your dog with a full meat diet, incuding its dog treats. While your dog might 'like' veggies and sugar and spice, these are mostly human interventions.
Naturally your dog would choose meat and be much healthier for it. The only thing missing in the picture above is a nice big wild fish for my dog to hunt!