That's a tricky one. Tides follow an approximate 2 weekly cycle depending on where the Moon is.
Spring tides are generally higher than neap tides.
So heights given on sea charts are given as the height above the average highest tide. You can also get mean sea level on sea charts. However, their mean sea level isn't the same as the mean sea level that the sea charts use. Mean High Water is midway between mean high water springs and mean high water neaps. Mean Low Water is midway between mean low water springs and mean low water neaps.