Water, barrier or highway?

We look at maps, and we see neat boundaries where the land meets the blue sea. The British Isles, surrounded by water, North America with vast oceans on each side, we look at the map and see a blue barrier against invasion.
Actually, water is a highway. Cargo, passengers, invaders, explorers travel by water. Water transport is cheap, and fast. Until the coming of the steam railroad, water was the fastest way to go, and it's still fast enough to compete against even jet aircraft.
England suffered one water borne invasion after another, starting with Julius Caesar, going thru the Anglo Saxons, the Vikings, and the Normans. Only when the English Crown could field a Navy was the realm properly protected. As late as 1778 Yankee privateer John Paul Jones could put landing parties ashore in Merrie Old England to take hostages.
Prior to the railroad, cities had to be port cities because only by water could enough food be brought in to feed even a medieval city. Ancient Egypt's cities brought their food in by Nile river boat. Same goes for ancient Babylon. With out the Nile and the Euphrates, the cradles of civilization would have suffered Sudden Infant Civilization Death Syndrome (SIDS).
The oldest cultures were based on rivers, because rivers are easiest to navigate, no tides, land is never far away, and you can drink the water from the river. Not til later would navigation of Homer's wine dark sea be mastered, leading to the brilliant Cretan and Greek civilizations. The stormy North Atlantic would not be mastered until Columbus.