Pilots are local navigators who assist ships in their maneuvers through dangerous waters. In theory, a sufficiently trained Sailing Master with perfect knowledge could safely take a ship through any given shallow area, but nautical charts are never perfect, and even the biggest, most elaborate, and most complete chart is no substitute for a veteran sailor who knows the depths, currents, and idiosyncrasies of a particular set of shallows, at low and high tide, in the summer and winter, and in the wind and rain.
Among the most common types of pilots are harbor pilots, who will wait in a pilot boat some distance off shore and, with permission, board ships that seek to make port. Harbor pilots are usually in the employ of whatever person or institution controls the port, and they are often chosen for their qualities as ambassadors as much as they are for their sailing expertise.