The best pics we have from medieval boats are on the Bayeux Tapestry:
And when you look at a ship from between 1500 and 1600 (which is past-medieval)
you still don't find any navigational lamps.
As far as I remember, even in the beginning of 19th century, there weren't more than just a single kerosin lantern on board of ships.
Thuis, I doubt that fishing boats had any at all - especially in mediƩval times (when you even didn't find a lamp in all houses).