It seems likely that you either have a short to ground , or you have a wire to wire short circuit in the water heater. I would turn off the power and troubleshoot with an ohmmeter.
With the wires disconnected from the heater, I would measure the resistance of either heater wire to a metal pipe on the water heater or another good ground that is not part of the electrical system. Each should be something over 1 megOhm normally, and it would be something less than 10 Ohms to blow the breaker. The same applies to the terminals on the water heater.
A wire to-wire-short would be a bit harder to detect. I would expect the resistance between the disconnected hot wires of the water heater to be more than 40 ohms for each isolated element, and more than 20 ohms if there are two elements.
Assuming you track the problem to the water heater, with ohmmeter readings in hand, I would try phoning the manufacturer or seller.