Practical Sailor had an interesting article about a laser device used
as an alert/locate device for ppl in distress. Basically its a 5 mW
laser diode with a plastic line generating lens. They say that it has
to be held to sweep the horizon and jiggled slowly and that it takes
practice before it is needed. Otherwise it worked great. I imagine
it might be hard for a person in a sinking boat to do this in spite of
practice so why not.........

Mount the laser in a housing with a weight so it always points toward
the horizon. The laser would also be mounted on a flexible rod that
would vibrate at the correct period when thumped by a spring loaded
hammer (a tuning fork like thing).