=== Opening a ship tunnel ===
Look for a [[rocktile]] tile that is approximately two-thirds below the [[Water#Water level|waterline]]. Needing to [[dredge]] such a [[tile]] is not unusual, especially to make the surrounding terrain look natural for those concealed ship docks.
The important thing is ''not'' to tunnel the ship entrance from the surface. You mine ''to'' the surface from inside, thus leaving an opening deep enough to sail through.