For long term SHTF water is one of the most important prep areas. I have been a developer for over 30 years and can tell you that if the electricity goes down all water heaters will be full. Water heaters work on the cold water going in to the water heater so the presser of the cold water has the hot water go to the bathroom etc. If you cut off the cold water the water heater is still full of water so you will have 30, 40 or 50 gallons of water in all the water heaters.
You will have to filter it after many days go by but it is water that many will not know about.