I think lasers (like cat toy or lectures' pointers) would be useless at 1mW: beam is too narrow and mal-focus to widen the beam drops the very low optical power.
It is a very interesting issue. I read one rowing club rules, one states:
- Lights – bright white lights that shine 360 degrees. These may be 2 lights each shining 180 degrees mounted at the bow and stern. Make sure the boat is visible from the side. In addition a solid red light should be carried on the stern. Spare batteries should be carried and the lights should be tethered to the boat to avoid them being lost overboard. In the event of a light failure the boat must not be moved until the light has been fixed or replaced.
My first guess is that existing lights (hopefully LED) on the "boat" could be modulated and that a rather simple phototransistor + amp + filter + alarm circuitry would provide an early warning. Filter & alarm could be a uC.