You mention the current rating of the SSR, but you don't mention the current that you are controlling. You do not say how your PI is controlling the SSR (are you using a derivative method, for instance?) or just on when it needs to be on and off when it is hot enough?
The heating elements in water heaters are typically made from a ni-chrome material and can pull large currents when powered, depending on the wattage rating of the heating element. Do you know the wattage rating of the heating element?
Some SSRs from China have visual ratings that are much higher than they can actually handle, is this a cheap Chinese Ebay SSR?
I have many more questions, but you could simply show what you have done and how you have done it in schematic and explain the theory of operation of your PI controller.
Also, there is the danger aspect of what you are doing. I have seen SSRs explode once they reach a point, so touching the SSR while operating is not a good idea, on many levels.