The fairwater is a structure above the main hull of a submarine that serves to streamline and protect the conning tower. If the submarine does not have a conning tower, the structure is known as the sail (US) or fin (UK). On top of the fairwater is the bridge which is only used when the submarine is surfaced.

The terms conning tower, fairwater, and sail are often (incorrectly) used interchangeably.

                        
                             
                         |   |
                         | | |
                      -------------
                      |  (Bridge) |
                      |           | <Fairwater
                      |  (conning |
   -------------------|   tower)  |------------------
  /                                                  \
 /                                                    \
(                                                      )
 \                                                    /
  \                                                  /
   --------------------------------------------------
           

(based on original ASCII art by bigmouth_strikes)