The Bloated Floattavern is a stationary boat in the harbor that activates a quest when stayed over night at. Also, the Thieves' Guild has regular meetings every night at the small region known as the Garden Of Dareloth. It is the site where the first Thieves' Guild quest is given.
The Imperial City Waterfront is the home of some of the poorer residents of the Imperial City. The shacks these people live in compared to the polished stone of the city proper is a great indicator in the disparity of wealth and perhaps a reason for the Thieves Guild activity in the area.