units.txt defines `water` like this: water := gram / cm^3 // Standard density of water (defined) 1 g/cm^3 is the maximum density of water at standard

units.txt defines `water` like this: water := gram / cm^3 // Standard density of water (defined) 1 g/cm^3 is the maximum density of water at standard pressure (1 atm), which it reaches at 3.98°C. (Or at least, that used to be true by definition. Using the current definition of the kilogram, and the latest measurements of water density, the maximum density is actually closer to 0.99997 g/cm^3.)