What does tide datum mean?

It is the reference elevation from which tide heights are measured at a station.

Mean Lower Low Water is the average of the lower low water height of each tidal day over the official datum epoch.

What does a negative tide height mean?

It means the predicted water surface is below the selected datum. It does not mean the area is dry or safe to cross.

No. Tidal datums are local station references, and different countries or products may use different chart datums.

Confirm the station, time zone, units, datum label, predicted highs and lows, and whether real observations are available.

The official sources used here do not establish that informal rule. Use the actual station prediction and observation instead.

Understand tide-chart datum, MLLW, negative tide heights, and why the zero on a NOAA prediction is a local reference rather than dry ground.