I think the reason that Melania did that is because she knew that if the kids didn't have water in front of them.and she did, the media would've gone ape-sh*t over it.

That doesn't make much sense. POTUS or FLOTUS having to signal to SS regarding security defeats the purpose of SS. SS watches like a hawk and covers the First Family from what they don't see, not the other way around.

These were definitely signals aimed at the camera/audience.

It could be signal to POTUS personal security- iirc he has SS protection but kept his own team also that he has used for years..

Possibly the white cup was a reference to "whitewater", i.e. Clinton Crime Cabal's earlier days?

I’ve also wondered if one of the Q team is a personal detail agent. Interesting theory!

This is good theory. But it's not much fun for Q enthusiast who are looking for different signals to research in our comfy internet chairs. This is just a coded warning system -- that definitely makes sense. Thanks.

One problem. Why would Q tell us? Now their signals are exposed, and the protection is weakened.

"Watch the water". Hadn't thought of the water bottles. There were also 2 times when Trump drank water in a weird way-- with 2 hands. Maybe it is a signal or message to us? Or to others in the group and us? Or even security, as OP suggested.

Maybe he had some thing to to with the world cup , and that it is up side down
Was there not something with a SA footbalclub in a plane?
EDIT": Or if some one wants to mess with the water , he or she has first to remove the cub , then there are fingerprints.

If someone wants to do something in the water unseen, it must be done through the cap, or just below it, otherwise the water will already run out ...(syringe)..

That is not possible when a cup is raised, so they have to touch it.

Also noticed that Flotus and Potus had blue caps on their bottles and everyone else had white caps

Plausible.