Dangerous to use water heater which activates circuit breaker?
It is probably &quot;safe&quot;. However, a problem which results in an overcurrent trip on one breaker might be causing damage to the related equipment (heating element, relays, etc.) in a way that doing so repeatedly (even for a week) might result in other parts of the heater breaking. Nearly impossible to tell, especially over the internet, as there are a lot of variables involved.
If it is possible to run the heater with that phase (leg) off, then that may be a temporary solution. However, depending on design that may not work at all, or it may result in the other heating elements running at a higher duty-cycle and cause other problems.
This is one of those situations where a few extra $ for fast shipping may be worth it.