The words "pier," "slip," "dock," "wharf," etc., have or had very
specific meanings.  A true "dock" is the same as a slip.  A dock, or slip, is
an inlet of water large enough to contain one or more ships.  A dock is
basically an enclosed area of water large enough to hold a ship; a dock is not
a wharf or pier, although it has now, incorrectly, come to mean that.
Manhattan had many slips in the 18th century.  These were replaced beginning
about 1790 with a system of projecting piers, or wharfs that projected into
the harbor to which ships could tie.  A quay is a wharf that is built parallel
with the water's edge, to which a ship can tie.   A wharf is basically any
built structure to which a ship can tie.

     I am curious to know why the word "slip," in the meaning of dock, is not
included in the Oxford English Dictionary.   It is, I think, clearly an
English language word.

Paul Huey