I'm not sure that anyone thinks that water is a threat to an airplane. The problem is that only the person carrying the water knows for sure that it is water. Doesn't matter whether you agree or disagree, the TSA's stance is that unknown liquids pose a threat. As every liquid that comes through security is unknown to the TSA, then every liquid is viewed as a threat, even if it really isn't. And tasting the liquid really isn't the answer as the bottle could have a fake bottom etc. Personally, I disagree wholeheartedly with the liquid ban, but I'm just saying that no-one, not even the TSA, thinks water is a threat to an airplane.