Reference level setting can be turned on from the settings screen.

This adds a reference line to your tide chart at the tide level you specify in the settings.

When to use reference line?

Reference level are useful to enable when there are particular tide levels you are watching for each day (for example: You can only visit a certain beach when the tide levels are at certain level or lower or you need to move your boat in or out based on certain tide level each day etc).

The exact times for these events are displayed on the chart at the intersection points with the reference line.