dead water

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 region in a body of moving water (or similar body of fluid in motion) where the water (fluid) is relatively stationary, or stagnant.