I've been zapped several times working on various electrical systems, and it appears to me that the danger of death by broken heater is overstated.
If your heater cracks, you would likely have at least both your phase and neutral wire in the tank. In a well designed heater, you would also have your ground wire in the tank, and they will all be close together. Most of the electrical energy will take the shortest path to ground, and will primarly complete the circuit by crossing the water and heading back down the ground or neutral wire.
When you place your hand in the water, you also may represent a path to ground, depending on factors such as what shoes you may be wearing, what your floor is, etc. Depending on how good of a ground your body has, you may experiance nothing, or you may experiance a small shock or tingle.
However, all of this aside- I cannot say their is zero risk of death, only that death is unlikely, and easiely avoided by simply unplugging the heater. And then throwing it away- Almost every electric shock I've experianced can be traced back to someone either not marking equipment as bad or reactivating deactivated circuits without checking why they were deactivated in the first place!