On bigger ships you may have to embark as passenger to get the option to dredge, because as the commander you might technically be too far way from the boat to use it. It is also a good idea to [[mooring anchor|moor]] your ship or boat before dredging.
You can dredge all kinds of material, for instance [[clay]]. However, clay, [[peat]] and [[tar]] tiles will only lower if the corner you are digging at is close to some terraformable tile type. The material dug is generally determined by the tile you are closest to its northwest corner.
=== Flatten or level ===