Your wire size is determined by the load of the particular appliance. The breaker is determined by the wire size.
14 gauge - 15 amp
12 gauge - 20 amp
10 gauge - 30 amp
8 gauge - 40 amp
6 gauge - 50 amp
and so on and so on.
You are more than likely over burdening your wire which is very dangerous. Age of the water heater? An electric water heater is a resistive load so it would heat up your wire and the breaker is doing exactly what it is supposed to be doing. Your wire is not.
That's the reason not to run an electric space heater on an extension cord. Same theory.
