Surprise! Phantomarine is... a substance!
A phosphorescent mineral/gem unique to this world, used in the lighthouses specifically to ward off seaghosts. To those who guessed this correctly, congratulations! You SMART :D
A more common, similarly-colored mineral called Revenite exists, but its glow is not self-sustaining. Which makes it a bit less... reliable. But still handy for the commonfolk! Personal lanterns, small boat-lights, and residential lighthouses all contain Revenite. The big main Lighthouse Roads - as mandated by the church and the goddess - are all Phantomarine.