Tide tables
When the water comes in, and when it lets you back out.
The water comes and goes
Tides are the oldest clock we have, and the one that still runs the coast. High water, low water, and the hours between — predicted years ahead.
What you get
high/low exact to the minute height meters above datum springs the big tides, twice a month neaps the small ones between
Why they matter
A harbor you can enter at high tide may be a shelf of rocks at low. The table isn't a suggestion — it's the difference between anchored and aground.