Cancel red alert, secure from general quarters. I figured it out.
All this time, I have never known where the water heater actually is.
Apparently, it's behind that metallic-looking plate with the exhaust port. I just now saw the little fingertip opening mechanism, I opened it up, and lo and behold, there was my water heater. And there, as advised above, was the white rocker switch (thank you Petefoss).
It was on, I turned it off. Problem solved? Stay tuned....