Allot of units with electronic ignition on the water heater have the switch to turn on the gas valve and activate the igniter located next to the water pump switch on the tank level indicator panel. I've seen where the lettering is worn off the switches. If there are two switches on the indicator panel, I would turn both on and listen for the peizo electric igniter to start clicking on the W/H.
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)