Slack water is the time during which no appreciable current in flowing in a body of water. Slack water usually occurs during both high and low tide, when the direction of the currents are reversing. Because slack water is usually determined by the tides, it can be accurately calculated in most regions.

Slack water is of particular interest to scuba divers. The absence of a current means that less effort is required to remain at a given site, and there is less likelyhood of drifting away from the boat. Slack water can also provide better visibility, as there is no current to stir up sand and silt from the bottom. It is very common for dive operators to plan their dives around slack water.