The short answer is it all depends. If the water was on because the company had to test the water retention of the pool/leakage, or other issues, then the amount of water needed or length of time on should have been discussed during the time period of entering into the contract. If the work was done while you were not home and you did not arrange to have a caretaker take care of the matter by chaperoning the pool company, then it may be your issue. This is of course if you did not include in the contract anything about being away and ensuring the pool company turned the water off. Now this is all of course if the custom, pattern and practice of pool company is NOT to turn on the water or turn it off. If it is custom, pattern and practice to turn the water on and off, then this is the pool company's negligence and they most certainly should pay the water bill.