I know why water was chosen. Brine also has characteristics that are useful. My point was that it's just one (specific version of a) molecule at one specific altitude, and pressure varies, so there is no true "sea level pressure". It is not a concrete thing where zero means zero.

Metric I love. C is not metric though. It's an SI unit, but not metric, as often as those are conflated. It doesn't have any fundamental advantage over F.