Common estimates based on the amount of water tied up in continental ice sheets during the last ice age put the Sea Level during the Ice Age at some 300 ft. lower.
If you think about human settlement patterns, that would put the largest trade hubs and cities of the time out on the Continental Shelf, provided such existed.
The climate change and rise in sea level (what the AGW people are so terrified of now, even if it isn't happening) would be rough on a civilization, but not insurmountable if the civilization recognized what was happening and moved inland.
Of course, conflict over real estate would ensue, just as it has at any time one group of people invaded another, and much as it might today if, say, the populations of the East and West Coast cities were to attempt to move inland to elevations higher by 300 ft. or more. That would be the biggest impediment to the survival of a culture based on oceanfront property. Loss of coastal plains croplands would likely have an adverse effect on population as well.
The AGW crowd imagine that most modern humans would stand in one place and allow the ocean to rise up around them.