If you currently have an electric resistive water heater, a heat pump water heater with the same heating capacity will use 3-4x less power, which means you can use a much smaller circuit.

A 6kW 240V EWH uses 25A, it’ll need #8 wire and a 35A or 40A breaker.

An equivalent HPHW would use 1.5kW at 240V, or 6.25A. You can use #14s and a 15A breaker.