If this was an Alligator Viewing Spot, and the public were being encouraged to come up to the fence in order to see the very numerous alligators who'd been corralled there, then I would agree that the fence is inadequate.
However, this is not a zoo. There's no invitation to "come see the alligators!" So, the fence is not acting to keep people and gators apart, but rather just as a simple visual reminder that the edge of the water is not suitable for swimming, wading, rinsing the sand off your toes, scooping up water for sand castles, or any other such activity. The nice big clear warning about gators and snakes makes it even more likely that most people will stay away (the mention of snakes, especially, should do it).
A random child ducking under the Grand Floridian ropes to jump into the water will almost certainly not die of a gator attack before he can be caught and returned to shore. Whereas a child ducking under the ropes at a zoo in order to dive into the gorilla pool is in much more immediate danger, and is much harder to retrieve safely. Different levels of risk, different requirements for fencing.