From you pictures, my best guess is that you have a dual source LP and electric water heater. The unmarked switch is probably to turn on the electric element in the water heater. Never turn this switch on without water in the heater or you will blow the electric heater element. The light on the panel marked "Water Heater" is for the DSI function for the LP burner on the water heater.
Rick and Janice Binns
2006 MS 32TK3, Mor/Ryde Pin Box and I.S., Auto Level-Up
2012 Ford F450 6.7 L Diesel, air bags and 25K Reese Signature hitch (factory installed)