Slack water, or slack tide, is the period during which no appreciable tidal current flows in a body of water.
Slack water usually happens near high tide and low tide, and occurs when the direction of the tidal current reverses.
For divers, the absence of a current means that less effort is required to swim to, and remain at a given site, and there is less likelihood of drifting away from a vessel or shore. Slack water can reduce underwater visibility, as there is no current to remove debris such as sand or mud. Except when drift diving, it is standard practice for divers to plan a dive at slack times.