We used a microphone to detect when the helmsman yelled.  When the helmsman yelled, the boat would begin to shoot water by turning on the bilge pump.  We used a microphone circuit bought from Jameco and hacked it to obtain a PWM-like signal.  This signal ranged from 0 - 7 V, which was too large to input into the LM339 comparator.  To get the signal into a range we were able to use, we fed the signal into a voltage divider made from a potentiometer.  We adjusted this potentiometer until the output was between 0 - 5 V.  Then we fed this signal through a voltage follower to ensure that there was no current coming in from the external circuit.  Then the output from the voltage follower was fed through a schmitt trigger made using an LM339.  The schmitt trigger was centered around 2.5 V and the hysteresis band was very tight (as we used a 100 K resistor).  The schmitt trigger allowed for a nice sharp square wave output.  When there was no sound, the line would idle high, and when the helmsman yelled into the mic, the line would go low.