I've posted this before, but I don't remember where.
Freshwater shallows in the Sunbelt states are a breeding ground for a particular bacteria that enters through the sinus cavity and aatacks the lining of the brain, causing symptoms similar to encephalitis. Very dangerous and oft times deadly. The water in RC was drawn from deep in Bay Lake through high volume pumps and gravity fed down through the slides. A floating bladder formed a spillway through which the water returned to Bay Lake. The problem was that much of the fresh water section of RC was shallow, thus providing a life sized petri dish in which the nasty little bugs proliferated. Add to that the propensity of kids (of all ages) to inhale, ingest, or otherwise allow that water to enter their bodies, and you have a recipe for disaster. The pool area of RC was deep- 10 feet, if memory serves. So, if you are thinking 'why not just open the pool' , then you are looking at high liability due to the depth. (Poly used to have a diving pool as well, but that's history, too) .
Regarding water quality, Bay Lake and Seven Seas Lagoon took some environmental damage over the years due to cumulative effects of fuel spillage, but they have improved recently as well. (This from a CM that I spoke to a while back) . That is why the lakes are full of life- from fish to snakes to gators. If you watch the birds of prey hunting fish, you will see that the ecosystem appears quite healthy. In addition, deep water activities are still advertised, such as water skiing and wakeboarding. It's the shallows that are potentially dangerous, but only if you submerge your face.
As far as someone dying at RC, I am fairly certain that did NOT happen. However, almost every year, someone is reported as dying from exposure. Last one that I read about was a couple of years ago in Florida. Two boys.