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?
Detecting object Water Reservoir
light-sensorsensingsensor
Related Solutions
A couple options come to mind.
Range finder
Use an ultrasonic range finder or, if you think the object would be opaque enough, an infrared proximity sensor. Put the sensor in the top of the box, pointing up and out of a hole you've cut there and poll it occasionally. When the measured distance decreases rapidly, and as a method of noise filtering stays low for a set number of samples, you've got an object in the way.
Properly placed load sensor
Another way is to put the load sensors on the bottom of the box. Assuming the object is actually placed on the box, the detected load will increase and can signify the presence of an object.
Strain gauge
Make the top of the box springy, that is, it's lifted slightly from a solid resting point. Now place a strain gauge in the gap. When an object is placed on the top it is depressed and bends the strain gauge. This signifies the presence of an object.
The above options don't necessarily give you a logical one/zero for presence, but neither would a optical sensor.
Momentary push buttons
If you're mechanically savvy, you could even use one or more push button switches. For instance, on the lid of your project box place three/four push button switches and attach (glue) a platform for the object on top of them. This method might not be terribly reliable though. I'd try to avoid too many moving parts.
Personally, I'd go with the optical range/proximity sensor.
A simple sensor used in such an application is a magnetic reed switch.
(source)
The simplest normally-open reed switch has two contacts, which are separated when there is no strong magnetic field incident on the switch. When the switch is exposed to a strong enough magnetic field, the contacts close and a circuit can be established through it. In your case, the float would have a small magnet on it, which passes close to the reed switch when the float moves.
The advantage of this approach is that no current flows, and hence no power is consumed, while the switch is open. Power requirement of the sensor is marginal even when the switch is off - just sufficient to overcome a pull-up resistor at the microcontroller sense pin, for instance.
simulate this circuit – Schematic created using CircuitLab
Best Answer
The easiest way would be to use a simple accelerometer, since there may be background noise like wind,etc. Just put it on a floating little buoy of sorts.