I managed to build a harbor on natural shore. I havent been successful on 'RAISED LAND' shores. Then again, for my successful harbor, my initial townsquare was 3 grids away from the shore. Whereas my other cities, (with RAISED LAND spell) they were much further away, but I was building my city towards the shore. Might want to test out if city was built 3 grids away from RAISED LAND, wehether it might work.