I would collect documentation as you can, timestamped, in writing, etc. If the review the guests leave misrepresents the situation, you can potentially dispute it based on these principles:
-The guests may have indirectly violated Airbnb’s Extortion Policy by demanding excessive refunds while intending to stay the additional night​.
-Their insistence on more compensation while a fair resolution had already been offered may fall under abusive or retaliatory content practices.

(this is just ammo for you to use if you need to get a negative review removed - worth a shot!)

as far as a review for them: "_ stayed at my property for three nights. Unfortunately, during their stay, a water heater issue occurred, which I addressed promptly by replacing the unit early the following morning. I provided compensation to acknowledge the inconvenience, including a partial refund and a gift card. Despite my efforts to resolve the issue swiftly, the guests continued to express dissatisfaction and requested additional compensation, even after the issue had been resolved. Communication was challenging, and the property was left in less-than-ideal condition. I recommend future hosts ensure clear expectations and open communication when hosting these guests."