Your installer is trying to pressure you into buying something.
What do you suppose happens to fireplace flues. I have two of them that are only rarely used, and they are in perfect condition after 41 years.
Winter air is dry because it won't hold much water. You might get some superficial condensation in your flue when it is raining with 100% humidity outside, but it won't soak into fired flue tile.
Replace your water heater when it needs to be replaced.
You can get a water/leak detector for a few dollars, and have it operate an alarm. Water heaters don't usually rupture with great flows. They leak. If you are concerned, I would go with the detector and alarm.
I suspect that a solenoid valve big enough for a water heater will be more than $100. They have systems for washing machines that might apply.