Hi all,
the electrical codes differ in different countries. Consult the
current one in your country.

One is that the Radio Shack tester can detect certain forms of
mis-wiring, but there's at least one which it cannot detect. This is
one in which you have a three-wire outlet, hooked up to a two-wire
power feed (one which doesn't actually have a ground), and the
installer simply wired the "neutral" and "ground" screws together at
the outlet.
This setup was legal and widely used in Czechoslovakia for many years.
Now it should not be used in new or reconstructed instalations, but it
is still in use in thousands of houses and flats. It doesn't seem
hazardous provided that the network is fused properly.

...
The other reason I'd discourage wiring the power-outlet ground to your
hot-water-heater pipe, is that the National Electric Code doesn't
permit this. If a short circuit occurs somewhere on this circuit, it
would cause your water pipes to carry some of the current back to the
house ground, and this could result in a shock hazard for anyone in
contact with the pipes (e.g. somebody in a shower or tub).
The Czech code requires to interconnect all tubing and conductive
items in a bathroom and connect them to the protective wire
(protective AND NEUTRAL in older installations, see higer)...

BR from Ivan