the basic density of water is 1kg per litre and petrol is about 0.785 per litre so the fuel will have gone to the top of the tank and the water will have settled on the bottom so all your getting through is neat water with a slight residue of fuel. so yes you will have caused damage, and the water will not be flushed through hard enogh even if you do clear the tank and try flush it through so you will need to change all your fuel lines and the tank probably, but could get away with flushing the tank and fitting a good filter like they use on aircraft fuelling called and fws which holds 9 coalescer filters to get rid of the water. basssssiccccaly your fooked