The sail is a structure above the main hull of a submarine that serves mainly as a streamlined fairing around the masts and antennas. On top of the sail is the bridge which is only used when the submarine is surfaced. If the submarine has a conning tower, the structure is known as a fairwater. In the UK the sail is known as the fin.

The terms conning tower, fairwater, and sail are often (incorrectly) used interchangeably.
                        
                             
                         |   |
                         | | |
                      -------------
                      |  (Bridge) |
                      |           | < Sail
                      |           |
   -------------------|           |------------------
  /                                                  \
 /                                                    \
(                                                      )
 \                                                    /
  \                                                  /
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(based on original ASCII art by bigmouth_strikes)