If they do ban people from bringing water into the parks, they will end up with a huge class action lawsuit. Not because they don't have the right for such a restriction but due to the heat in Florida. All it will take is a few people to start passing out from heat stroke and the lawyers will find a way to make a gigantic class action out of it.
I can certainly understand (and support) restricting food. Humans can go without eating for a whole day in a theme park without major issues (unless you have diabetes or some other medical condition). Also, there is a cost to cleaning up after people that bring food in and to dispose of the trash.
Same thing for non-water drinks. Nobody can argue (again, with the exception of medical conditions) that they need to drink soda, juice or milk while they are in a theme park.
However, becoming dehydrated from lack of drinking water is a major problem for every human. You can not safely spend a day at a theme park (especially when it is hot) without drinking some water. Water fountains are not sanitary. If they ban water from coming in, the only way they will avoid being sued at some point will be to provide convenient water dispensers where you can fill a free cup with water.
Also, unless they institute pat downs on the way in, I can guarantee you that I can get past security with a bottle of water on my person.