Depth sensor / Pressure Sensor (easy to use with arduino)
This is an easy to use depth / pressure sensor. This model works up to 100PSi, so about 70meters depth. They do also have 1000PSI versions. The sensors just takes an input of 9 to 20v and outputs between 1 and 5v, depending on how much PSI it detects. So just connect that to a analog port on the arduino.
The sensor is available via digikey: http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=US&KeyWords=msp-300-100-p-4-n-1&x=0&y=0