The method of knowing the approximate water surface position from the topography of the coastline as seen from the sea is called "Yamadate".
If there is a feature on the shoreline such as buildings, chimneys, islets, roots, boat mooring buoys, pontoons, etc., or places with a lot of diving experience, it can easily be used to find out the approximate surface position.
First, set two easy visible points (a palm tree and a mountaintop building) that are as far apart as possible from the coastline.
Next, set other two easy visible points (a hotel and a church at the summit) that are as far apart as possible from the coastline.
The water surface position is determined.
In order to do as accurately as possible, make sure that the two marks on the same line are as far apart as possible and that the two lines intersect as vertically as possible.
Let's use the ones that are as far apart as possible, such as "palm trees" and "buildings on the mountaintop".
