You have this large water reservoir, for example, a bathtub. How can you detect when an object is thrown (rock, etc) into a big water reservoir? Vibrating sensor, ultrasonic etc?
The easiest way on my opinion would be to catch the waves on the surface of the liquid(water). Could b easily done even with a mechanic switch anchored to the side at the surface level and a piece of Styrofoam, which following the waves will move up and down and turn the switch on and off, or at the bottom and via a string and a piece of styrofoam floating on the surface and pulling and loosening the string/cord while waves are present .