I'm sure the "height of liquid column " is not so bad. In fact we talk about the force that a column of liquid exerts on the base, divided by the area of base. There are formulas to handle it, in SI the pressure associated with a column of high H is density*g*H.
In hydraulics this unit is still working. A centrifugal pump is usually designed to give the same "H" (discharge head) no matter it pumps water or gasoline. A lot of notions are "heads" in pumps terminology- suction head, NPSH but also in estimating pressure loss when using Darcy equation.
I guess in API 650, Pi is expressed in inch of water by historical reasons. Previously, API counted only the roof plates as helping the roof under pressure and it was quite attractive to count this effect as "8*th" because th inches of steel are equivalent with 8*th inches of water (by densities ratio).

About kg/cm2 as pressure unit, it is in fact kgf/cm2, an old non-SI unit of pressure- technical atmosphere (symbol: at), and kgf and kg as units had the illusory advantage to use the same number for mass and weight on Earth's surface of that mass. Gives the same confusion as lb and lbf (as weight unit) vs the second law of dynamics.