You need to consider the power consumption of the water heater in relation to the size of the main circuit breaker.
The heater will have a rating or classification plate that will show it's consumption in watts or kilowatts.
Take the rating in watts and divide 250 (volts) into the figure in watts and the result will give you the rating in amperes.......... if that figure is higher than the size of your main circuit breaker, you need to get a larger main circuit breaker.
If the heater is something like a power shower that draws a lot of power, it might not work on the local (as opposed to the main) circuit breaker.