The water heater are 4500W, so you would draw about 20 Amps. Breakers are usually sized at 1 and 1/2 times the rated amps. Doubling the overload will remove some of the safety feature and may violate code. There might be a problem element or a decaying breaker; it is better to replace the bad part than to defeat your safety feature.
Dont increase your amp setting, the idea is that the fuse blows, if you raise the amp then the chance of heating up the cable to the unit increases, and may cause more problems, get a electrition to look at it