It is possible to get water out of a bowser if the refuelling dept doesn't do its job properly. You get condensation inside the tanker which lies under the fuel (greater density). The operators are supposed to drain this water off. Incidentally, same applies to aircraft tanks. In my experience, the FE will normally do a manual drain if the system is not designed to do it automatically.
If you've got a spare hour or 2, visit the fuel dept - it's fascinating what they do to the stuff we put in our tanks, and how they do it. I think you would be re-assured on the quality control issue.