Ultrasonic transducer for fish finder - TD02002
I haven't done any fish finders, I've played with ultra sonics.  (I don't know what frequency they use for fish finders, but maybe one of the manufactures spec sheet will tell you.)
Anyway, you want to first hit it with a big voltage pulse.  Maybe one or two cycles at it's resonant frequency.  (most new signal generators will have a burst mode that you can use.)  This sends out a sound pulse in the water.  Then you are going to use the same transducer as a detector... looking for the reflected wave.  An opamp will probably help here, but to start you should be able to just plug it into a 'scope input.  Now you've got to do some math... look up the speed of sound in water.  Figure out how long it will take for the pulse to bounce off something and come back.  You want this time to be much longer than the burst that made the sound pulse... 'cause your transducer (and any amp) has to recover from the large drive signal.  I'm not sure a bucket will be deep enough.  Maybe you can launch the wave's horizontally in a bath tub and put a board down into the water to reflect the wave.