In the UK the fin 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 fin 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 US the fin is known as the sail.

See sail for an ASCII diagram.

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