I like the idea of running lights on the boats, but it would only be practical in friendly waters. As was already mentioned, running lights would be shut down on ships during wartime to reduce visual acquisition range at night. This was also why U-boats received orders to consider any ship not operating its running lights as hostile.
On a more sarcastic note, U-boats did come with a "light" of sorts. The problem was that the "light" was called METOX and one could only see it from Allied radar sets.
