The nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180° meridian except where it is interrupted by territorial waters adjacent to land, forming gaps: it is a pole-to-pole dashed line. Ships are required to adopt the standard time of a country when they are within its territorial waters, but must revert to international time zones (15° wide pole-to-pole gores) as soon as they leave territorial waters. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours.
In reality nautical time zones are used only for radio communication etc. Internally on the ship, e.g. for work and meal hours, the ship may use a suitable time of its own choosing.