Assuming you’re talking about a detached, single-occupancy home, and not an apartment, condominium or other such structure as @elbanditoroso mentions, and if you have no isolation valves for the water heater, then you’d need to shut off the next closest upstream cold water supply, and that could very well be the “whole house” shutoff. In other words, you may need to shut off water to the entire structure in order to isolate the water heater (or other fixture or water-supplied appliance).
If you do repair or replace the water heater, then that would also be a good time to add a shutoff so that you can isolate it in the future without having to shut off the supply of water to the whole house.