What amperage braker for a 240 hot water heater?

The power rating and voltage for the pump will determine the size branch necessary to supply the power to the load and the over-current protection.

For instance, a 1 hp pump would be 746 watts. At 240 volts, that would be a little more than 3 amps, when operating, and possible 10 amps during the first half-second when it starts up.