It is possible to do land reclamation by placing a ship containing engineers on an ocean or lake tile, and then ordering those engineers to transform the tile into swamp.
However, I have noticed that not all water tiles can be transformed into swamp. Sometimes the transform to swamp button for the engineers is blanked out. From experience, only some ocean/lake tiles next to a land tile can be transformed; it is not possible to transform an ocean tile in the middle of a vast ocean into land.
What are the exact conditions under which an ocean or lake tile can be transformed into swamp?