Apparently it is being reported that a resort nearby - the Hyatt Regency has a similar lagoon and they post the alligator warning. Years ago I stayed at this resort for a business convention. I do remember the lagoon and the warning does ring a bell but I can't say for sure.
Many tourists do think that alligators are only in the everglades or alligator alley - unless they see warning signs.
There are lots of things tourists are not familiar with - it doesn't make one stupid - in London for example, they post signs at cross walks to warn you to look left - why? Because many tourists come from places where the traffic is driven on the right side of the road. They aren't stupid just used to looking right when crossing the street.