What if we make a land building transferring building adjacency to naval factories? Like in 1 adjacency building per 1 naval factory. You build it on land, connect it to a naval factory, the building receives adjacency bonuses from economy buildings surrounding it and transfers them to a naval factory (but not too far, has to be close to water).
As of now, naval units do not get any discounts. A little discount may make naval battles more interesting.