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.^[1] Tide tables indicate the time of high and low water at ports and other locations. Slack water can be accurately calculated in most regions using a tide table or current table combined with either a tidal atlas or the tidal diamond information on a nautical chart.^[2]

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.

For sailboats with limited top speeds, a favourable current can substantially improve the vessel's speed over the bottom for a given speed in the water. Difficult channels are also more safely navigated during slack water, as current may set a vessel out of a channel and into danger.

In some parts of the world, slack water does not coincide with high water or with low water. This occurs at many river mouths or where a large body of water opens to the ocean through a small outlet, such as The Rip between Point Nepean and Point Lonsdale, Victoria at the entrance to Port Philip Bay, Victoria, Australia or Gibraltar at the entrance to the Mediterranean Sea.

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