Electric water heaters don’t draw that much energy…
If you keep getting brownouts like you are an even blackouts its most likely going to be something going on between your house and the pole like some have already said, its odd for your house not being that old and having this problem(my house is over 100 years old and is still in better shape that a lot of newer homes). My guess would be its at the service drop…saw a few houses like that last year where the power would surge and sometimes go out completely for a period of time