Or his girlfriend on the side. Or someone she hired to do the job. She has claimed that she did not realize he was married during their "relationship". But if she found out and realized that he was not going to leave his wife, this would be the perfect nasty revenge -- kill (or have someone kill) his wife, and frame him with the evidence.
The problem with any theory about a frameup is that sinking the body near his fishing area is a bit too neat. After all, who would have known that the fish wouldn't have devoured every scrap? So any frameup theory is immediately pretty weak.