I would build a little circuitry to detect the proximity of a boat on the sea.
On my boat I have a reflector and I would add a little antenna (not a big radar antenna) to recept other boat radio waves (I think it is 2.4GHz) and alarm me when I am slepping...
You schould have something like a frequency scanner with a trigger circuit.
Take a look: h**p://www.aceco.de/products.htm or h**p://www.optoelectronics.com.
They have the ready made scanners/recorders.
Try using the following system:
Amplifier (Monolithic cheap amp), 2.4 GHz filter, again 30-50 dB amp. stage, then a simple diode detector, and some DC comparator circuit.