Good idea, but it's troublesome to change to bigger DB Box and find the neutral wire dedicated for the water heater and need also separate the neutral bar for just water heater.
For simplicity i just change the socket or plug with built-in RCBO according to usage amphere. All of my water heaters and air conditioners are with RCBO either with socket or plug. Much easier

I have this 10mA but only 13A for water heater.