What is MLLW?
MLLW is the average height of the lowest low tide of each day over 19 years. NOAA uses it as 0 ft on tide charts.
Because it is an average, some low tides go below it. Those are shown as minus tides.
Charts and boating maps use the same zero, so a depth on a chart plus the tide height gives the water depth.
Tide FAQ
Why do some tides show minus?
They are lower than the average lowest tide, so they are below 0.
Is MLLW the same as sea level?
No. Mean sea level is higher than MLLW.