Slack water usually happens near high tide and low tide, and occurs when the direction of the tidal current reverses. Commonly available tide tables indicate the time of high and low water at popular locations, such as ports, but not at every specific area that a navigator may be. 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.
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 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 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 dangerous shoal water.