Answer based on context:

The incorporated city fit into 4-5 square miles as late as the First World War, but has since greatly expanded, crossing the Ashley River and encompassing James Island and some of Johns Island. The city limits also have expanded across the Cooper River, encompassing Daniel Island and the Cainhoy area. The present city has a total area of 127.5 square miles , of which 109.0 square miles is land and 18.5 square miles is covered by water. North Charleston blocks any expansion up the peninsula, and Mount Pleasant occupies the land directly east of the Cooper River. Charleston Harbor runs about 7 miles southeast to the Atlantic with an average width of about 2 miles , surrounded on all sides except its entrance. Sullivan's Island lies to the north of the entrance and Morris Island to the south. The entrance itself is about 1 mile wide; it was originally only 18 feet deep, but began to be enlarged in the 1870s. The tidal rivers are evidence of a submergent or drowned coastline. There is a submerged river delta off the mouth of the harbor and the Cooper River is deep.

How much of the city's square miles was not land?
18.5 square miles