I think these fences and the new signs are a good move.
I am a cautious parent and a rule follower, and I could have made a similar mistake to this family. The scary thing is that I DID try to do research on possible risks at WDW, but still was very ignorant.
I read up on bed bugs, how to prevent my kids getting lost, how to keep contact info on them just in case. I read up on child predators, stroller thieves, ride safety, pool safety, balcony safety, etc. I knew alligators populate Florida, I've been to the state a dozen times and have family there, and I also knew they sometimes wander onto Disney property. But I was very ignorant on how frequently and how many were on property, and that they would be so close to human activity and noises at the resorts.
We stayed at WL last year and the beach was closed for construction, so we never went on a beach. But I wouldn't have known it risky to allow my 2 year old son to kick some of the water at the edge, or if my 6 year old had gotten his hands sticky from marshmallows and wanted to rinse them off quickly (followed by anti-bacterial). I wouldn't have known they were breaking the no swimming rule by just touching the water, because that warning means something different in the places I've lived.
With these fences and signs in place, I of course wouldn't have let my kids anywhere near the water, even in the daytime. These are a very good deterrent for people like me, who want to practice good safety and follow the rules, but just need the necessary information.