Technically they're called 'Lightships', and they were made to do the work of lighthouses. Your crew can't even see them, which is why each one has a tugboat close by to act as a tender. When your crew calls out "Ship spotted" they're seeing the tender and not the lightship itself.
http://en.wikipedia.org/wiki/Lightvessel
http://images.google.com/images?sour...ed=0CBsQsAQwAw