From what I understand of the X-ray machines my hand luggage goes in, they are only able to reveal the shape of objects, especially if they are not made of metal or other X-ray-opaque material. So I assume that airport security officers (TSA, or its European/international equivalent) can only scan them for items that look like bottles.
Is it so easy to see?
It is so easy to see, if they wish to.
It is not so easy for you to see if they wish to see.
It is exceedingly unwise to even think about doing what you ask about.
The object of the exclusions is to eliminate items which have a reasonable prospect of being energetic enough to constitute a significant explosion hazard to the aircraft (I am informed by a customs person who gave me far more detail on what their expert lecturer had told them that the lecturer would have been happy with. (The bad guys know this). They go to considerable effort and expense to do this and put passengers to considerable inconvenience as well. You can be sure that they would take great satisfaction in demonstrating to you the ability of their equipment to detect a "waterskin".
Modern detection equipment can detect the presence of a wide range of materials quite selectively. Whether an airport has the equipment able to detect any given type of material is not something they are liable to willingly make public - but trying them out is one way of finding out. Water and compounds with a significant water component are able to be detected selectively using nuclear magnetic resonance techniques. Similar methods are able to detect various high explosive compounds and are able to distinguish between various distinct types. I do not not how many airports have equipment able to do this but once you could handle TNT, adding water compounds would probably not be difficult.
I have passed through airports with anonymous 'boxes' located on either side of the path through security. I examined these as well as I could (I'm an engineer :-) )and I'm reasonably sure that they were explosives sniffers.
Over a decade ago I purchased in Germany a replica drinking horn modelled on a cows horn. (A present for my son who is a mediaeval re-enactment enthusiast). It took me some considerable effort to determine that it was made of plastic. Visually and by feel it was extremely hard to tell. On passing through NZ incoming inspection I did not declare it as it was not biologically based.
Agricultural officer on the incoming inspection machine.
"Have you got a cow's horn in there?" Me - "Yes, but it's plastic".
He "Oh. I wondered why it was the wrong colour!".
Their machine assessed materials and highlighted biological matter in a different colour on their display, as that was what they were interested in.
If NZ can pick out a plastic cow horns and distinguish it from a real one over 10 years ago, a waterskin should be a doddle.
Risk depends almost entirely on the people concerned, what sort of day they have had, what they think of people from your country or ethnic group for whatever reason and what they consider your risk profile to be. You would be placing yourself entirely in their hands and consequences could range from nothing at all through confiscation, an instant fine (eg NZ $200 minimum if you have eg an undeclared apple), court appearance, refusal of entry, worse.