Your water probably has a standing pilot so you would not be able to turn that off and have it come on automatically the only way I can see to make it happen is to buy a timer with either a 110v or 220v motor and set it for the times you want and use the switch contacts to break one of the wires between the thermostat and the gas valve so pilot stays lit but gas valve not allowed to open until the contacts on the timer close. I have never heard of anyone doing that but I gues it is possible. The best thing to do is invest in a tankless system and then you only heat when you use the water.