Noun
a stretch of water without current or movement
the occurrence of relatively still water at the turn of the (low) tide
Source: WordNetslack water
suddenly they were in a slack and the water was motionless Source: Internet
Ice sheets typically form along the shoreline and in slack water bays, where the lake is not as deep. Source: Internet
The moment that the tidal current ceases is called slack water or slack tide. Source: Internet