So, why not to add a "searchlight effect" to the main lighthouses ?
With a turning light beam (i've added a rotation controller for that) ?
You can even change the color :
Maybe, we can use this feature to add different colors for the light, depending on the angle ?
However, more tests are needed to see if that's possible.
The side effect is that the beam on the sea surface can only appear if the lighthouse has detected an ennemy unit ...