It's also worth noting that the waters where depth accuracy is most important to a submarine are also the places where the depth is more likely to change (estuaries, ports, especially new/expanding/busy ports).

Tides & currents moving the bottom (sand, etc.) around, mostly. But it's also possible for a coral reef to have grown taller. Or a volcano, I suppose. :)

NOAA's Chart No. 1 explains the symbols used on charts.

https://www.nauticalcharts.noaa.gov/publications/docs/us-cha...

The broken assumption thee is that the ocean floor doesn't change. Things are changing all the time. Faster and faster as you get shallower and shallower.

Shipping channels and shallows need to be resurveyed constantly or you have no idea what you might encounter.