In reply to "wonderings" query:
There is no "normal" water content level for Jet A-1. Ideally it should have no water in it at all. When attending each aircraft, fuel company staff should do a water check (this is in addition to water checks carried out at the fuel storage facilities)at the commencement of refuelling. This is done by inserting a water sensitive cartridge into a fuel sample which then changes colour if water is present. The exact figure escapes me but I seem to remember from my apprentice days that water content should be no more than six parts per million. Can anyone offer the exact figure? Obviously water can still find its way into fuel through mishandling or condensation in the tanks and this is why fuel tank sumps are drained on a regular basis during routine maintenance(water being denser than fuel will sink to the lowest point in the tank). Large transport aircraft also have fuel scavenging systems which draw fuel from low points in the tanks and feed it to the inlets of the booster pumps thereby removing any water that may be present before it freezes. I don't have experience of biz-jets, but if they do not have such systems, this may be why they have to add anti freezing fluids to the fuel.