In certain coastal regions, a rat is a hazardous stretch of water where strong tidal currents collide with submerged crags or rocky outcrops. These areas are notorious for creating unpredictable eddies, overfalls, and standing waves that can easily tear a small vessel apart during stormy weather. Navigators actively route around them when the tide is running against the wind.
Heard on the job
Keep well clear of the rat on the ebb tide, or she'll get chewed up on the crags.