If the situation was that Ray drowned himself—possibly swimming miles downstream first—and his body simply wasn't found, then it will likely never be solved except by blind luck (if a fisherman reels in a human skull, for instance). Even that wouldn't truly solve the case because we wouldn't know for certain if Ray's death was a suicide; we would know that he was dead, however.
If Ray was murdered, then it won't be solved unless somebody talks—and that hasn't happened in 17 years—or, again, by blind luck. Any potential perps could easily be dead themselves at this point.
This case is not going to be solved based on the information that's already in the files, in my opinion.
Drowning victims can be difficult to find, even in shallow water. Some float to the surface within a few days, some float to the surface within a few weeks, and some never float to the surface. The ones that do float to the surface remain mostly below the surface and are difficult to spot during broad daylight let alone at night. Some become entangled in large masses of debris that collect in certain locations along the bank. Some become covered in silt.
In my opinion, a suicide by drowning is an order of magnitude more likely than foul play, and foul play is an order of magnitude more likely than any remaining scenario.