The Coastal Seas as Ancient Highways to the Land
Several established and increasing by numerous pieces of evidence seem to indicate that sea-going craft (at least ideal for coastal journeys) were a surprisingly early invention of Mankind. For, hunter-gatherers, constantly on the move to find food, would be certain to regularly encounter rivers across their migratory path, and though searches would have been undertaken for places where they could be forded, the possibility of crossing water immediately via easily constructed 'boats' would have saved a great deal of time and risks. Indeed the Channel Islands, off California, have revealed fossil human bones from a very long time ago indeed, and these could have only been reached by boats of some sort.
Although much later historically, there is the fact that Orkney included a very advanced early human settlement, involving an integrated group of stone-built houses, in an area devoid of trees. The inference was that such a settlement was not only close to their main source of food, the sea, but also directly upon the "main highways" of that time linking many well spread settlements accessible by boat. The land was certainly not endowed with any rich source of game, and was totally unable to support any real agriculture.
Finally, the problem of exactly how America was initially populated with the first wave of human beings to actually get there, both from Asia and perhaps even Europe, would be much more simply explained by, if the peoples concerned were competent with boats, and feeding themselves almost exclusively from the sea. Read more on this here
Now the clinchers for archaeologists will always be the finding of such ancient vessels. But, surely the earliest boats, without metal tools, would have to have been made with materials such as skins and branches, or birch-bark and resin, with perhaps a wooden frame of some sort. Such vessels would never survive so many thousands years as such, but the skills in making them would be maintained by groups where they were constantly needed. The beauty and functional superiority of the North American birch-bark canoe is a masterpiece of excellent handling, lightweight and easy to repair. Such consummate boat building would require a long period of development to arrive at such a masterpiece.
Since the melting of the ice sheet at the end of the last Ice Age, we know that most coastal areas, just where these derelict boats would be, were vastly and permanently inundated by the rising seas. Current underwater archaeology is severely limited by both poor visibility, the difficulty of disposing of spoil, and the kit necessary for people to effectively work under the sea. Without doubt there must be some vital traces under the sea bed, and near river mouths, preserving evidence of our maritime past, preserved deep in the sediments.