They do monitor the water in their parks, everyday. It's at the resorts where the issue occurs. A lot of their land is used as a natural preserve, so to police and monitor those areas would take a large amount of individuals and time.
Just to put it into scale, the Walt Disney World Resort is around 43 square miles of swampland with industrial development within it. A comparable city size to that would be San Francisco.
Yep, I think WDW employs close to 70,000 individuals alone and is on natural preserve land partly on loan and shared with local stakeholders. It literally is a city unto itself.
Many should keep in mind that part of the negotiations for the expansion of WDW(over the years and particularly recently), is that Disney has a contractual obligation to maintain and preserve the wildlife and presence of the land they inhabit. Essentially they can't just mass murder or expunge these creatures, fuck with the soil and habitat, or do any harm to damn near anything. Even regulations on construction for stuff like the Pandora park undergo years of planning and negotiations for the tiniest details. WDW actually is very serious about following protocol and legislation to a T on this stuff, but gators exist all over and aren't a problem so easily dealt with.
There are obscene filters in place through more lawyers and city/state government representatives than you can fathom during the development process, but I get the scrutiny in this situation and it was horrifying to all. Yet, I don't see this quite so much a fault of improper prevention, so much as it was life finding a way.