My wife has said it for years while going to Disney, that the announcement Cast Members make about keeping hands inside the vehicle, is not a suitable safe guard to prevent an accident. This guy could say that while loading into the vehicle, there were a number of distractions that prevented him from hearing the announcement, such as a kid crying, Brazilian tour group chanting, etc.
If it did go to trial, Disney would have to prove that they did all they can do prevent an accident from happening and the announcement of keeping hands inside the vehicle is not all they could have done. Putting Rails on the boats would be the most they could have done to prevent an injury.
On an off topic side note, speaking of Dog Bites. My wife prosecuted a man who had a dog and a sign that said "beware of dog". Two neighborhood kids, uninvited and basically trespassing, hopped a fence to run through peoples backyards and were bitten by this guys dog. The owner lost 50,000 to each kid for the bites. The guy had to prove he did all he could do to prevent an attack from his dog and the judge said Barb-wired fencing is the most he could have done. In Disney's case, the same thing applies, they have to prove they did all they can do to prevent an injury.