To me, the rule vs. danger argument is nullified by the fact that "no swimming" has an implication of danger. Why else would WDW prohibit swimming, other than because its dangerous? Whether the danger is alligators, snakes, no lifeguard, bacteria, a sharp drop off, rocks, etc., it poses a risk to the point where Disney has a rule in place. If you choose to wade in the water regardless of that no swimming rule, you're at risk for the same dangers that a swimmer might face if they ignored the rule as well.
Not blaming the parents by any means, my only point here is that I think the signage was appropriate. Could the signs have said "do not enter water" to be more concise? Sure. But how can any company have the foresight to predict every common sense error humans can possibly make?