Chart datum

A chart datum is the water level that depths displayed on a nautical chart are measured from. A chart datum is generally derived from some phase of the tide. Common chart datums are "lowest astronomical tide" and "mean lower low water". In non-tidal areas, e.g. the Baltic Sea, mean sea level (MSL) is used.
A chart datum is a vertical datum and must not be confused with the horizontal datum for the chart.
Choice of tidal phase.
The following tidal phases are commonly used.
Lowest astronomical tide.
Lowest Astronomical Tide (LAT) is defined as the lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions.
Many national charting agencies, including the United Kingdom Hydrographic Office and the Australian Hydrographic Service,
 use the LAT to define chart datums. 
One advantage of using LAT for chart datums is that all predicted tidal heights must then be positive (or zero) avoiding possible ambiguity and the need to explicitly state sign. Calculation of the LAT only allows for gravitational effects so lower tides may occur in practice due to other factors (e.g. meteorological effects such as high pressure systems).
Mean Lower Low Water.
The United States' National Oceanic and Atmospheric Administration uses Mean Lower Low Water (MLLW), which is the average height of the lowest tide recorded at a tide station each day during a 19-year recording period, known as the National Tidal Datum Epoch. MLLW is only a mean, so some tidal levels may be negative relative to MLLW; see also mean low water spring. The 19-year recording period is the nearest full year count to the 18.6-year cycle of the lunar node regression, which has an effect on tides.
Mean Higher High Water.
Similarly, the Mean Higher High Water (MHHW), is the average height of the highest tide recorded at a tide station each day during the recording period. It is used, among other things as a datum from which to measure the navigational clearance, or air draft, under bridges.
Usage.
Charts and tables.
Charted depths and drying heights on nautical charts are given relative to chart datum. Some height values on charts, such as vertical clearances under bridges or overhead wires, may be referenced to a different vertical datum, such as mean high water springs or highest astronomical tide (HAT) (for "HAT" see tidal range).
Tide tables give the height of the tide above a chart datum making it feasible to calculate the depth of water at a given point "and" at a given time by adding the charted depth to the height of the tide. One may calculate whether an area that dries is under water by subtracting the drying height from the [given] height calculated from the tide table.
Using charts and tables not based on the same geodetic datum can result in incorrect calculation of water depths.
Satellite navigation.
In recent years national hydrographic agencies have spearheaded developments to establish chart datum with respect to the Geodetic Reference System 1980 (GRS 80) reference ellipsoid, thus enabling direct compatibility with satellite navigation (GNSS) positioning. Examples of this include Vertical Offshore Reference Frames (VORF) for the United Kingdom Hydrographic Office (UKHO) and Bathyelli for Naval Hydrographic and Oceanographic Service (SHOM).