"During the last glaciation, sea levels were almost 300 feet lower than today." 300-500Ft, most accept 400ft.
"I heard about a dredging in British Columbia that picked up some stone axes, etc, from the bottom of a channel/sound. "
Yup. That was something. They calculated where human settlement would most likely have been during the Ice Age and went dredging there. Amazingly, they found human artifacts.
I have some 7,000 year old wood dredged from Santa Rosa Sound in Florida that was once part of a coastal forest.
sea level 300-500 ft. lower.
The continential shelf off the mid-Atlantic states today runs 60 miles out to sea. Much of it is the submerged coastal plain, the '20 fathom flats' off Virginia. The 'edge' near Norfolk Canyon would have been ocean-front property 20,000 years ago.
The Chesapeake Bay was then a shallow valley of the Susquehanna river, running another 60 miles out to the ocean through the forests. Incidentally, some of these forests (stumps) are uncovered by storms along the shore.