That amount of sea level increase can hide a lot of real estate and sea shore property is where the money is made, you know, ports and shipping.
Marine archeology is in its infancy now, but I believe we will in the future be reading more about lost cities underwater and the civilizations that have disappeared under the waves.
Even today there have been reports and photographs taken of stone pyrmids deep off the shore of Okinawa.
The jury is still out on whether this is manmade or not. Otherwise....I agree 100% with what you've said. (Actually, I hope that they are manmade)