Chart datum is not sea level
Three datums appear on one small chart, and mixing them is a quiet way to run aground.
What chart datum is
Chart datum is a low-water reference chosen so the tide almost never falls below it. Heights printed on the chart are measured up from that surface, which sits below mean sea level by roughly half the local spring range. Treating it as sea level makes every depth look generous.
Where the second datum appears
Gauge readings and harbour notices often use a local zero that is neither chart datum nor mean sea level. The offset is a local fact and belongs in the port index beside the reference port, not in your head.
The sign is the whole job
Most datum errors are sign errors. A shift applied in the wrong direction doubles the error instead of removing it. Write the shift with its sign on the plan and show the arithmetic once, so a second reader can reproduce it.
The rule worth keeping
If a height has no stated datum, it is not a height for a depth decision. It is a number that happens to look like one.