slack water

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A period of cessation in the strong flow of a current of water, especially at high or low tide.
2. n. An area in a sea or river unaffected by currents; still water.

GNU Webster's 1913

1. adj. the time when the tide runs slowly, or the water is at rest; or the interval between the flux and reflux of the tide.

WordNet 3.0

1. n. the occurrence of relatively still water at the turn of the (low) tide
2. n. a stretch of water without current or movement