When were the first "boats" built?
The oldest discovered boat in the world is the 3 meter long Pesse canoe constructed around 8,000 BCE , but more elaborate craft existed even earlier. A rock carving in Azerbaijan dating from ~10,000 BCE shows a reed boat manned by about 20 paddlers. Others argue that hide boats (kayaks) were used in Northern Europe as early as 9,500 BCE.
However, the very first sea-worthy boats were most probably built long before that, about 800,000 years ago, not by Man but by his predecessor Homo Erectus, a smart naked ape.
Erectus originated about 1.8 million years ago (Mya) in Africa and the species survived for over 1.5 million years before being supplanted by Neanderthals and our own Homo Sapiens. Erectus had learned how to put cutting edges on stones and perhaps how to control fire; but he lacked the gene for speech and language.
Erectus lived through the Ice Age(s), a period of repeated glaciations, each lasting about 200,000 years, separated by warm spells. At the extreme, glaciers covered up to 30% of the world's land mass and tied up huge volumes of water resulting in the sea level dropping 100 m or more. This created temporary land passages between islands and continents. At some point, England was connected to Europe, Siberia to Alaska and parts of Indonesia linked to mainland Asia.