[HELP]Interfacing a Fish Finder's Transducer Sensor

So even an IR Proximity Sensor will have a difficulty in determining the depth of water.. ? I'm desperate, How far can an IR proximity sensor penetrate underwater, considering more or less 2 ft would be sufficient to me,

look at this:
http://philippines.rs-online.com/web/p/photoelectric-sensors/6666564/

can you suggest anything with this sensor??