In Reeds, harbours in the first paragraph of their description have ML and then a number, understand this is Mean Level but what is this, why is It published and how do you use it.
Thanks K
See http://tidesandcurrents.noaa.gov/datum_options.html
Mean Tide Level
The arithmetic mean of mean high water and mean low water.
So half way between!
How do you use it? - I never have.
Mean levels Hi or Lo are just an average, so of no practical day to day use.
I'm fairly confident that someone will now come along and tell you how important they are