I am giving recommendation on 3kwatts load (to be on a safer side). For 3kwatts of load with Voltage of 220V, the current is approximately 35A.
So if you are using a relay look for one that can handle upto 35A.
The TRIAC is recommended more than the relay because: relays are electromechanical in nature so they make noise when switching, and their reliabilty can not be compared to that of semi conductors like the TRIAC.
You know water heater is a resistive load so zero crossing is not neccessary.