Maybe it was Smart Water. Maybe a preferred brand of sparkling water that UA doesn't serve. Maybe electrolyte water. Maybe a liquid medicine that the pilot was prescribed that tastes vile but is tolerable mixed with juice. Maybe it was any other clear, non-alcoholic liquid that the pilot desired or needed. Who knows (other than the pilot)? As I mentioned upthread, these sorts of situations can be life altering for a pilot if a misinformed accusation is leveled against them. We can debate about the wisdom/appearance of a pilot in uniform pouring a clear liquid into a glass of cranberry juice in public, but there's no way I would ever accuse them of violating UA's rules about alcohol based solely on visual observation of a clear liquid being consumed.

Here's a simple test: If a uniformed pilot seated in coach pulled out a water bottle and drank a clear liquid from that bottle would you report it to the crew or to UA? I'm fairly certain no one would. Yet somehow the act of mixing that same clear liquid with juice becomes a huge potential issue/offense.