Implementing Safety Shut-Off Relay In Project
Do not implement safety systems in software. If you are relying on this circuit to prevent a dangerous situation, i.e. electrocution or fire, then you had best use something more reliable than a bit of Arduino code to assure your safety.



Here is a (modified) example of something I once designed to protect a submersible pump. These pumps live under-water so it is important to detect if water is leaking into the electrical parts, which would be bad.



Notes:


Relays rated for large currents (i.e. anything over a few amps) are not called "relays", they are called "contactors". Contactors are rated for full line voltage (230 VAC) and large currents (the Schneider CT series covers 16-100 A.)
this system is self-resetting, i.e. the power will be cut off automatically in case of water being detected, and power restored automatically once the water leak is no longer detected. You can arrange a latching relay of some sort, in series with the contactor coil, if you desire a hand-reset latch.
The Flygt MiniCAS II is the first example of a leakage alarm that came to mind. It is an industrial product and probably overkill for your needs. However, "water leakage alarm" is a good thing to search for.