"Is it naive of me to consider my water heater as a 40 gallon reserve water supply?"

Nope! But...

Do you ever flush the mineral deposits out of the tank? When I first moved here, I attached a hose and opened the spigot. The water could hardly come out because of the buildup of debris. I had to change to a 5/8" hose, still had trouble, and finally just disconnected the hose and poked in the opening with a screwdriver to break up the chunks.

Check it before you need it.

Sue