In Florida freshwater lakes, including those on Disney property, there is are rare brain-eating amoebas, which can cause death. High water temperatures and low water levels are perfect breeding conditions for these Naegleria Fowleri amoebas. They are what supposedly shut Disney's River Country Water Park, and are still found in Disney lakes today. Since the discovery of these amoebas in Disney lakes, guests have been banned from swimming in them.
If you listen to the DISUnplugged podcast, which is part of the DIS network, you may hear Pete Werner say "Stay out of the damn lakes" at the end of each episode - this is why!!