Latest research places early humans in Africa camping on the beach or in caves and living off the sea's resources. Just as some monkeys do today, just as did Neanderthals in Europe and the earliest migrations out of Africa that followed the shores to Australia and beyond. Early migrants into N. America did the same; sails, paddles, boats or rafts may have been one of the earliest inventions, not one of the last. Imagine the coastal voyages, the leapfrogging of small groups along long stretches of coastline, and then think how knowledge of wind patterns, ocean currents, the migration routes of birds, driftwood arrivals from windward and the paths of moon, sun and stars, must have been developed into sophisticated bodies of navigational knowledge, horded in the memories through song and poetry and passed on to succeeding generations. Like, say fifty thousand and more years ago.
This photo of children at the beach shows how easy and natural that must have been. Playing at the shore, wading, swimming, fishing, beach combing, are one of our most natural activities.