Your water heater should run on propane regardless of any other appliances (stove, refrigerator) connected or running at the same time. If it does not operate correctly, it's possibly due to (1) it being in the "electric" mode, (2) no 12V power (some models) (3) some switch not in the right position, (4) pilot light not lit (some models).
If you are trying to remove the stove and still have the water heater operational, you will need to turn the propane off, disconnect and cap the gas line to the stove, and turn the propane back on.
If you are having problems with gas flow and hence the water heater won't work, chances are you opened the propane valve at the tank to fast. In which case you need to close the valve, bleed some propane off (like light a stove burner for second or two), and re-open the valve very slowly.
If none of these are your problem, you need to be more specific about what you're trying to do, what is -or- is not happening, if everything worked fine before, and/or if this is a first time problem.