It is a method to know the underwater position by underwater terrain.
In places with a lot of diving experience, it is possible to determine the underwater position fairly accurately.
The easy-to-use marks are as follows.

●Terrain and structures

Great obstruction, rock, drop-off, artificial reef, wreck, etc. are good landmarks.

Drop-off

Artificial reef

Wreck

●Geology

The geology changes towards the offshore and becomes "Stone" → "Big rock" → "Sand".

●Creature

Fishes that ives around rock do not travel a long distance in a short time.
As corals and soft corals do not disappear in a short time, there become good landmarks.

Coral

Soft coral

●Ripple mark

Ripple marks can be made parallel to the coast.

●Visibility

The visavility changes due to the inflow of rivers.