The RCDs protect against a ground fault, not an overcurrent situation. Even a small current going to ground will trip the RCDs. It is most likely your heating element in the water heater, but could also be the thermostat. I guess there could also be a fault in the wiring to the water heater, but that's unlikely.
I'm not an electrician, but here's how I was told to troubleshoot:
I have a Quick Nautical Boiler. Check both the thermostat and heating element for continuity between the line and ground, and between neutral and ground. If you find any connection, thats the culprit.
If it were an overcurrent problem and tripped the breaker, you would check the resistance between line and neutral on both the thermostat and the element. On my Quick Boiler, the heating element should show ~40 Ohms. Sorry, I don't remember what the thermostat should show. If it differed significantly from your new spare thermostat or heating element, that's your culprit.
Hope that makes sense, please correct me if I've got something wrong here.