slack water

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the occurrence of relatively still water at the turn of the (low) tide
• noun a stretch of water without current or movement