Not much, since the ocean floor tends to be quite young on geological terms. For example, the oldest parts of the floor of the northern Atlantic ocean only started to form after Laurasia broke apart 50 million years ago, and the floor of the Pacific ocean is regularly recycled in the subduction zones of the pacific ring of fire.

Right, but organisms are small, get eaten, rot etc. Civilizations change their environment, build cities, manufacture tools with stronger materials. Advanced civilizations even more so.

Likewise Sundaland, whose mountaintops are now Indonesia; and Sahul, between Australia and New Guinea. Australians have oral records of the upheaval that came from that flooding.