Real ales in casks (wood or aluminium) are easy to add water to, but as real ale fans are very critical these days it would be risky as it would be detected & word would get round. As Rtaxron says, if one has a keg spanner it is possible to add water, but one would have to depressurize
the keg first. It is possible that you had the first drinks out of the line that session, or that whoever cleaned the pipes either didn`t rinse them through properly or didn`t get all the rinsing water out. There is an "ullage" allowance & I always drew off 1/2 pt from a keg & 1 pt from a cask (varies depending on the length of pipe from cellar) serving the 1st customer. As to the draught coke, it is mixed at the tap from a box of "syrup" coke & fresh, chilled water, so yes, you will see the 2 separate liquids blending to a metered mix.