Don't worry about "violating" GAAP. no one is going to jail b/c you didn't accrue this bill. Here is what i would do. write a memo about what happened, make a good faith estimate of what I think the water company will actually require me to pay and accrue that. attach the memo to the bill and attach both as backup for the accrual. if the water company comes back and requires me to pay the original amount, then i expense it at that time.
Dont leave your management in the dark about this bill. tell them what you are doing.
Caveat: only do this if you are not crossing fiscal years. as long as all this takes place in the same year, you'll be fine.