This isn't a good option either. It would be possible to have a bottle that was lined in such a way as to have drinking water and an explosive in the same container. Or, perhaps the explosive is denser than water and non-polar... if this is the case, you could add enough water to make it drinkable, and still have the explosive available.
Hence the asshole comment. You can always find a way to smuggle a bomb onto a plane. Limiting case: swallow a condom full of plastic explosives and shit it out in the restroom, the same way drug mules carry cocaine or heroin. There's no real way to stop this short of X-raying every traveler.
The questions the TSA should be asking are:
1.) What is the likelihood that a potential terrorist will use this means of smuggling weapons on board?
2.) How effectively can our screening procedures catch weapons, given that they exist?
3.) How much inconvenience does this cost passengers?
People had little problem with metal detectors and X-rays, because 1.) your clothes and carry on luggage are the natural hiding places for a bomb 2.) the machines will catch most of these instances and 3.) they're not a huge hassle for travelers. But by the time you get down to chucking water bottles because there might be a liquid explosive in them and it might be denser than water and it might be non-polar - really, what are the chances that a terrorist will use that means of smuggling? Compared to, say, overvolting a laptop battery?
For that matter, I think metal detectors may also be obsolete, now that it's fairly easy to make plastic/composite knives and guns.
You wouldn't even need to swallow it. Just strap it to your body, as it won't set off the detectors. You can assemble the detonator from parts in the rest room. X-rays are simply nowhere near 100% effective. Similarly, you can easily make knives or limited-use guns from non-metallic parts.
If we believe the official story 100%, then they merely plotted to do so, but never actually attempted it (they did, however, according to the official story, 'smuggle' non-volatile liquids on in a dummy run, without actually risking taking anything they'd be in serious trouble being caught with).
So, the best case scenario is that no-one 'did try just this'.
That's ignoring all the many holes with the case (that ended up requiring using a legal backdoor to convict them without a unanimous verdict.
You could also have a hidden bladder of fluid inside most anything. That'd make much more sense than trying to hide fluid in a portion of a fluid container.
And, indeed, that's just what I said to the security goon before he dragged me into this interview room officer. I really don't understand the problem!
