I had quite a bit of experience with this whilst I was at sea both in fuel and lub oil- usual cause was bacterial growth at an interface between oil and water - cure was a cocktail of chemicals which isn't much use to you now.

It is possible that the contamination has been present in your fuel tank for some while and only manifested itself at the last fill up - did you run your tank unusually empty at any point.

Having said that if you could arrange for the fuel stations storage tanks to be indipendantly dipped with water finding paste to show if water is present - then if it is getting a sample of fuel from the interface tested would show if the problem was theirs or not. Establishing their co-operation might be difficult though.