If you have this appliance, and the pilot light is lit, you are still using energy even if the appliance is not turned on. Please check the check box and it will automatically calculate the monthly and yearly cost for you. Pilot lights typically burn 24 hours every day and the average BTUs/hr is 750 or 5.48 therms/month.
There is not a specific section for you to enter your water heater usage because it is covered through all of the appliances listed that use hot water. Please also note that not all water heaters use pilot lights.