In sailing, HAT stands for Highest Astronomical Tide. This term represents the highest tide level that can be predicted to occur under average meteorological conditions and any combination of astronomical conditions.

Significance: It is the highest water level predicted by tidal models, not including storm surges or extreme weather events.

Usage: Used by navigators and mariners to determine maximum water depth for coastal navigation, clearance under bridges, or to assess tidal ranges.

Context: It is part of the standard tidal calculations and nautical charts, often referred to when ensuring a vessel will not run aground or when planning for maximum vertical clearance.

Other "Hat" related terms in sailing:
Cocked Hat: A triangle formed on a chart when three or more position lines (bearings) do not intersect at a single point, often caused by errors in observation or movement of the vessel between taking bearings.

Sailor Hat/Cap: A brimless or narrow-brimmed cap, often called a watch cap or "miki," designed to stay on in high winds and keep the ears free.