I wish that one had been mine, but I have to give credit to the comment on this DeadSpin thread, which features a super-slow-motion version of "The Gulp Heard 'Round the World" (or whatever).
For myself, the first thing that came to mind was the classic "flop sweat" scene with Albert Brooks from Broadcast News:
In all seriousness, what really made my jaw drop wasn't the fact that he had to take a swig of water--those lights are hot, and it was one of his first major national TV appearances, after all--but that he/his handlers were stupid enough to place the water way out of reach.
Here, Marco, take a look at this picture of the King of Public Speaking, Steve Jobs, during one of his famous keynote presentations; notice anything?
Seriously, here's a clip from one of one of Jobs' keynotes, where he's introducing the "Numbers" spreadsheet program. The swig is about 30 seconds in (thanks to BentLiberal for the find):
Rubio thought that it was more "professional" to keep the bottle out of sight and to keep his eyes locked onto the camera (or the teleprompter?) the entire time, as if this would somehow keep anyone from noticing his "deft" sleight-of-hand movement. Instead, by trying so hard to keep anyone from noticing what he was doing, all he did was make himself look like an idiot.
The only other thing I'll add is that what we actually just witnessed tonight was Chris Christie sealing the deal for the 2016 GOP nomination.
Anyway, here's my own tribute to Marco Rubio: