hun, personally i would be sitting down and writing them an e-mail mentioning the following things:
1. You have NEVER been requested to pay a water bill previously
2. You have nothing in WRITING stating that you are supposed to be paying water bills.

Just make it simple, dot points and ask that they please provide in writing anything that states you MUST pay the bill and if so when it was forwarded to you and if you signed it that you would like a copy.
If they say they DON'T have documentation that states you WILL or WILL NOT pay water ask why you haven't previously and what has changed now and that your happy to come to some agreement but that you would like it in writing.

FWIW we have ALWAYS had to pay for our water usage, landlord paid the rates side of things but we paid usage, is that not that case with all rentals???
add to this... to forawrd copys of what you signed with this information. that way they can't just whip up something to suit.