It is probably not a good idea to have the water heater on if the breaker
keeps going off. Unless you have a defective breaker, it is trying to tell
you something.

How many watts is the heater? How many amps is the breaker? Did it just
start doing this?
If you know someone with an ampmeter (or want to spend $30 for a really fun
tool) it would be nice to measure the amperage.