If Disney knew there were gators, the no swimming sign was just as easily replaced with a gator warning sign.
I'm sorry, but I see no good reason to think the water at Disney is any different than the water anywhere else in Florida, or Georgia, or Mississippi, or Louisiana or Alabama. It doesn't matter if Disney or a typical homeowner, or the casinos in Biloxi, or anyone else puts out a sign expressly mentioning alligators when one is in "alligator country."
Anyone with any sense doesn't need a sign; they need nothing more than situational awareness. It is not Disney's fault or burden to create situational awareness for them.
I don't even live in FL, and I know there are quite possibly going to be gators in the "natural" bodies of water in FL. I know there are possibly gators in the waters of the MS and LA bayou too, and I don't need there to be a sign telling me that is so. When I went to several African countries, I had better sense than to stand at the edges of rivers and lakes, and nobody told me it wasn't a good idea not to stand there are there were no signs.
I have cats, but I also know that foxes live in the woods adjacent to my neighborhood, and I don't want to risk the cats "getting into it" with a fox, so I don't let them out at night. I live smack in the middle of D.C. There's water next to the Disney resort -- water that's as close to the building as the woods are to my home -- and the mere presence of the woods is enough for me to suss what kinds of creatures may live there...fox, raccoon, opossums, and so on.
I live right between the two woods you see in the image above. I don't know when the fox is going to be around; we don't have a neighborhood fox watch. I know that it can be around; I know it has legs and will travel in the search for food. I know a fox, if it sneaks up on my dozing cat will find an easy meal. I know too the fox isn't waving a sign or blowing horns any more than is an alligator lurking in a lagoon to find an easy meal. (Which, BTW, the alligator clearly didn't, after it nabbed him, see the boy as dinner insofar as the boy's body was found intact.) And that's enough for me to take reasonable care with my cats.
Don't you think a somewhat greater degree of care is warranted with regard to one's child in a state known for having alligators in the water? You honestly don't think common sense would tell you, me, or most people that have some, not to temp fate by going into fresh (or salty) bodies of water in a place known to have alligators around?