Steve,
Radio Shack makes a Mini Audio Amplifier for $14.99 that would do the trick. It needs a 9 V
battery but a good tech could build you a resistor-9V Zenner diode
power supply that would wire into your 12 V ship's
battery and only draw a few ma of
current. You could use a microphone or wire it to the chart plotter speaker. How is that for
cheap and easy?