dont throw the tank of fuel away to start off with ...
poured a bottle of Methylated spirits into the tank and off you go...
about 500ml per full tank should do the trick ...
the water combines with the alcohol and burns of with the petrol so that could save you a bit of money there
as to the water problem itself .. could be a number of things causing it ....
i can only think of two right at this moment though
1) faulty petrol cap allowing condesation to bild up and collect in fuel tank ...
2) faulty head gasket on engine allowing water to engress into the engine ... and then go through the car with unspent fuel and into the petrol tank
hope this helps
john gibson
January 2010