The most popular story linked to the reason of it's closing it because the bacteria in the lake is a health hazard to people. The water park was made to be like a watering hole to swim in so they used actual lake water. Once they found out of this bacteria, they disallowed people to swim in it so it had to be closed. It's another reason why people can't swim in the seven seas lagoon or bay lake.
When I go down to FL we stay at the GF and I'll usually take out the water mice (sea raycers). While you aren't supposed to stop and take pictures when you are in these boats, I have a couple pictures of River Country in its current state - only they're on my phone and I'm not sure how to upload photos from it to here haha.