A harbour is a protected body of coastal water, accessible by ship. Harbours may be man-made (i.e., by dredging and the construction of a breakwater) or naturally occurring. Ports are almost exclusively situated in harbours. During storms, ships traditionally have sought out accessible harbours to provide safe haven.
Harbours have natural navigation hazards - the entrance is often narrow or non-aligned to prevailing wind & currents; as such, vessels must operate within a restricted space both to enter the harbour or move about within. Breakwaters are typically formed of boulders or large rock outcrops which can quickly sink a vessel. Vessels operating even at modest speeds will be subject to a number of hydrodynamic effects, such as ship squat, from the restricted waterspace.
When a ship is being designed for operations to particular harbours, the designer must be aware of the limitations imposed by harbour spaces. Vessels maneuverability will likely be decreased by shallow water. Beam and draught restrictions will be imposed by the harbour entrance. Pollution can be a corrosion issue. Water quality and salinity can have deleterious effects on water-service systems (i.e., cooling) and allowable displacement, respectively.