How to shut off water to a shower/tub with no accessible separate shutoff valves for cold and hot water.
Where is the main water supply shutoff?
Take off the shower/tub face plate and check to see if there are screw drive stops where the water comes in if so just turn them off. If you don't have these stops you have to turn off the water to the house. Your hot water heater should be okay, just turn the gas or power off then turn off the the valve to the water heater and you will be fine.
When your done then turn the water back on to the house then turn the water on to the water heater and run water to get the air out of the lines then turn the power or gas back on to the water heater.
You my need to take off the aerators on the faucets when flushing out the water lines because of debris in the water from closing and opening the water valves.