@tursiops , Yes I am aware that there are occasions when the tide can be lower that LAT, but these are expected to be exceptional cases.
In addition, as you correctly state, air pressure, and prevailing winds can also change the actual depth on the day. But irrespective of the datum and tide table used, these are factors that you would not normally be able to account for.
From a diving point of view, if wind and tide are having that much of an effect, I seriously doubt I would be diving!
@Storker. Thank you for your original comment. It has sent me on a search for knowledge.
As far as I have been able to find.
Chart Datum does change.
Charts for Tidal Waters use LAT (Lowest Astronomical Tide).
LAT is used by the UK and Australia, t
The USA uses MLLW (Mean Low Low Water), which is based on an average calculated over 19 years. What I don't understand about MLLW, this seems to be a moving figure, as it is a 19 year average.
Charts for non tidal waters (e.g. Baltic Sea) use MSL (Mean Sea Level).
Lowest Astronomical Tide is used because it is defined as the lowest tide level that can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. This ensures water is always deeper than specified on the chart other than under exceptional conditions.
Mean Sea Level. The average depth of water. However, there appear to be several MSL definitions.
The simplest, the midpoint between the mean low and mean high tide.
However, the global MSL, is used for chart datums and to calibrate altitude (aviation).
I will continue my education.
My thanks
@Storker