Tides on Earth are caused by the gravitational pull of the Moon (and to a lesser extent the Sun) on the oceans. The cycle is roughly 12 hours 25 minutes between high tides at most coastal locations, with two high and two low tides per day. A wristwatch tide-indicator complication tracks this cycle, typically displaying current tide state on a sub-dial or a dedicated indicator hand. The mechanism is mechanically simple: a slow-rotating disc or hand geared to the lunar cycle, advancing through a 12h25 phase per cycle.
Practical limitations: tides are geographically specific. The exact phase relationship between the lunar cycle and high tide differs between coastlines (Mediterranean tides are minimal; Atlantic tides are strong; Bay of Fundy tides are extreme). A tide-indicator wristwatch is calibrated to one specific coastline (typically chosen at purchase or set via the crown). Modern implementations include Hermès Cape Cod Tide (2020), Tag Heuer Aquaracer Calibre 7 with tide indicator, and various marine / yachting references. The complication is more decorative than functional for most wearers; serious yachters use dedicated tidal almanacs.
