I have a Furuno GPS
7000 plotter and I put a remote
alarm buzzer in for just the same conditions of which your are thinking. My Furuno has wiring
for an external alarm through the data wiring
cable which sends 12 volts down the cable when the alarm is activated. I went to Radio
Shack and got a 12 volt piezo buzzer and a 12 volt red LED which I mounted below with a switch to allow selective usage. Now I can set the alarm on the plotter, select the internal and external alarm on the plotter, turn on the switch and I will get an alarm and a flashing red light if the alarm is activated. The red light is nice if the engine
is running and the switch is nice because there may be other crew asleep when you want to activate an arrival alarm at a waypoint but do not want to wake the entire boat!