Looking at the charted depths on coastal nautical charts, what do they actually represent? Are they high or low tide or simply average depths?

http://oceanservice.noaa.gov/facts/sounding.html

Cannot sleep, so what better thing to have on my mind at 4 am!

on a NOAA chart they represent MLLW which is defined as "The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch."

So they basically take into account all low tides during the year. If you are boating in a area that is borderline for you best thing to do is take some actual measurements and compare them to the chart at low tide and see how it is. It is often possible for it to be shallower than what is shown on the chart.

pstorti, thank you for the reply and information, your description cleared up the definition.