There will be a lawsuit.
Disney deserves to be sued and lose multi-millions. I'd say anywhere from 10-50 is reasonable (I know it's a wide range).
Yes there are signs but none warn of alligators. Any attorney will ask "why didn't the sign say 'danger: alligators are in the water'" Imagine you're on the jury and the lawyer points out the sign says "don't go in the water" vs "alligators are in the water" and asks you, the jury "how would you behave differently if you saw sign #2"? This is a slam-dunk case of negligence on Disney's part.
Will things change? Nope. What can Disney possibly do? Drain the lakes and fill them with sand? Build giant fences all around every body of water? Talk about ruining the beauty of the area. The lakes, waterways, boat transportation, etc are all part of the draw to WDW.
Frankly, from a business point of view, one alligator attack every decade and a $50 million dollar lawsuit is nothing. Remember the mayor in Jaws? That's not a completely made-up reaction of a business person/politician to these types of events.
Yes this is tragic, but let's all keep our heads here. Bad things happen and we don't need to go drastically changing everything just because of one sad event. The movies on the beach will still happen, there will still be smores on the beach, they will still have the "don't go in the water sign" but no "danger: alligator" signs.