From what I read, the sign said "no swimming". No mention of alligators. And nobody probably saw it until it was too late, moving slowly with only the nostrils above water. There are waterways across the world with "no swimming" signs, that don't have alligators.
Aside 1: They used to entertain tourists in norther Australia by dangling a lump of meat off the back of a tourist boat for a croc to jump out of the water and snatch it. Unsurprisingly a croc did it one day when there was no meat dangling, and took a tourist. Fun fact: they almost always recover the bodies in crocodile attacks. They don't have teeth for grinding or tearing, so they hide the body till it starts to rot and they don't need to chew.
Aside 2: John Oliver: "I will happily embrace a Latin night at a gay club in the theme park of the world – as the ultimate symbol of what is truly wonderful about America.”