It has been said, but a little devils advocate non-legal, and personal opinion. Known material defects must be disclosed. If it were me, I would argue that no running water in a significant portion of the house, if known, is a material element of a purchase contract as it adversely affects both the value of the home (or requires an immediate, unforeseeable and likely significant remedy), and reduces the SF of livable space for X amount of time per year. Indoor plumbing is a material element of any home purchase contract. Structural elements of a home are not the only material elements of a home. Modern homes have plumbing, electricity, DWV, and mechanical systems (where necessary). Those are material elements of the home. If a portion of any of those were significantly damaged or unusable for an indefinite period of time per year, I would argue that is a material element that requires disclosure. Counter arguments exist, obviously, and I would guess that there are a lot of details to sort through. But, again, if it were me, I might articulate all that in a letter to the previous owner and politely suggest a meeting to discuss a remedy. They are under no obligation to do so absent of some compulsion (they clearly did not consider it a material element), so be very nice. It is also very possible that they are good folks who appreciate you buying the home. They may want to help!
Case law exists on the topic, I would imagine, which you can likely find on your own. Additionally, a quick look through the code book, and the online statutes pertaining to property law in your state could help. Not to ball bust them, but so you know your leal rights prior to engaging.
Or, call a lawyer. Lots of good property lawyers about, and they are not terribly expensive for simple issues like this.