Can I use a 220v AC rated switch for 220v 120Hz DC?
First let me say that I agree with all the comments made (Matt, Brian et al). 

My concern about this question is safety - mains voltage can and does kill. 

There are good reasons why only fully qualified and trained technicians and engineers should attempt to work with mains voltages in projects. The fact that so many questions are being asked about the comments made above, together with the poor quality of drawing and a fundamental error in drawing a simple bridge rectifier, does not fill me with confidence.

(1) The bridge rectifier you have drawn is wrong and will not work.



(2) A water heater element is electrically isolated from its metal casing and normally fitted with some form of thermostat (usually a bi-metallic switch) to prevent it getting too hot. The metal casing surrounding the element must be suitably earthed to prevent shock in the case of a fault. 

(3) Both DC and AC are able to give electrical shock. At 220V either can be fatal. (I'm certain we all remember the fate of topsy the elephant - http://en.wikipedia.org/wiki/Topsy_%28elephant%29 )

(4) A simple circuit only requires a single break to prevent current flowing. For AC the switch is usually placed in the 'live' wiring as the 'neutral' is considered to be near earth potential. An example of this would be household light switching. 

(5) Isolation switching is when the live and neutral lines are both switched off. The first switch to open switches the circuit off. It is not critical that both switches operate at exactly the same time. If you think the supply is shorting through the water then there is a serious (and dangerous) electrical fault.

(6) There is no advantage in rectifying the AC supply to feed a resistive water heating element. Each half cycle will heat the water and the the mass of the water will 'smooth' out the heating effect. It will not make it safer. 
Switching the element on and off is best done at (or near) the zero crossing of the input to reduce surge current. This can be done with a suitably rated opto DIAC/TRIAC (solid state) switch. For example -