dead water

noun2

1. •
   The eddying water under a slow-moving ship's counter, or a similar area of stationary fluid or gas in advance of a concave angle, which can occur when there is strong vertical density stratification (due to salinity or temperature or both), and can cause ships to become hard to control.

   • A "deadwater," that is a region occupied by stationary fluid or by gas at suitable pressure, ahead of a concave angle was shown to be possible by Villat. — 1967
2. •
   A region in a body of moving water (or similar body of fluid in motion) where the water (fluid) is relatively stationary, or stagnant.

   • Some of our party went down the deadwater for a short distance in it and came back with three black ducks and one golden eye. — 1905