Atlantic Ocean

The answer for this might seem simple, and part of it is; sailors were afraid of getting lost and never being able to return home.

The trade winds of the North Atlantic blow steadily west (counterclockwise); it is difficult to move a sailing ship up wind. In the mid=Atlantic, the Gulf Stream (an ocean current) and the air currents that mirror it moves in a clockwise motion, from the French/Spanish coast, down to the Mediterranean before turning back east. Unfortunately, the southern portion of the Gulf Stream is also a major storm track, making sailing difficult for smaller vessels. Attempting to sail straight across this section of ocean will land one square in the middle of the Saragossa, the large brackish area of the central North Atlantic. (Stay tuned, I'll come back to this)

The method of sailing upwind is called "tacking." The history of sailing in the Atlantic is not nearly as old as that of the Pacific, but it is difficult to nail down the time period when sailors first learned (or even what culture learned) this method of sailing upwind. The earliest known explorers of the Atlantic, the Phoenicians, did sail west into the Atlantic, and Himilco, a Cartheginian captain from the 4th century BCE is credited with discovering the Saragasso Sea (also called the North Atlantic Gyre) which lies roughly at the center of the Atlantic directly south of the southern most point of Greenland, on about the same latitude as the island of Cuba. The importance of this is that the waters and winds of the Saragasso are largely absent by comparison to the rest of the Atlantic Ocean, creating huge problem for those who are very wary of running short of necessary supplies, and who rely largely on wind and water currents to keep them in close proximity to those supplies.

The Norse sailing into the Atlantic a couple millennium later managed to settle Iceland and Greenland, and there is substantial evidence that they reached the eastern coast of North America, and returned to tell about it.

It bears mentioning the longstanding myth that early sailors believed the world was flat, and that sailing west into the open ocean, they feared falling off the edge of the world. This false bit of history has perpetuated for decades and is based on an error in translation for a history textbook from the early part of the 20th century. Ptolemy and the mathematicians and geographers who preceded him knew and proved before the first century BCE that the earth was spheroid. Columbus possessed Ptolemy's maps, and the mistake he made was based on Ptolemy's miscalculation of the circumference of the globe.

Consider that despite the presence of about 85% of the water on Earth, the world's oceans, as far as concerns humans, are massive deserts--that is the presence of drinkable water is absent. People without water can last at best 3-5 days, and after 1-2 days of water deprivation will require assistance to perform even the easiest tasks. Ships of the early days of ocean exploration were small, and storage being limited, they could not carry the supplies necessary for very long trips. This required that they sail within sight of land where they could acquire necessary supplies when needed. Look at any map of the western coast of Europe, and you will see how close villages, towns, and cities are--literally within a few miles of each other. Some of this rises out of the number of fishing villages they rose from, but more importantly, these were resupply stops for sailing vessels as well. Look then at the east coast of North America and you will see the same pattern repeated. However, as you move south, the distance between communities widens. Look then at America's west coast, and even today (aside from the sprawl of large metropolitan areas), you can still see large expanses of coastline with no villages, towns, or cities.

The argument can be made that the early Polynesians were able to sail for months without resupply, but unlike the Europeans, Polynesian sailors used vessels that facilitated their existing lifestyles, and were able to resupply on the fly, so to speak. That is they were able to fish as they went, and eating their fish raw, did not require large quantities of fresh water. The Polynesians, too, were not encumbered by the inability to sail or tack upwind. They sailed with the wind, and when it failed, or they desired to move against it, they rowed--much like the early Norse sailors.

The Amazon Basin, the largest drainage basin in the world, covers about 40 percent of South America, an area of approximately 7,050,000 square kilometres (2,720,000 sq mi). It gathers its waters from 5 degrees north latitude to 20 degrees south latitude. Its most remote sources are found on the inter-Andean plateau, just a short distance from the Pacific Ocean.

The Amazon River and its tributaries more than triples over the course of a year. In an average dry season, 110,000 square kilometres (42,000 sq mi) of land are water-covered, while in the wet season, the flooded area of the Amazon Basin rises to 350,000 square kilometres (140,000 sq mi).

The quantity of water released by the Amazon to the Atlantic Ocean is enormous: up to 300,000 cubic metres (11,000,000 cu ft) per second in the rainy season. The Amazon is responsible for about 20% of the total volume of freshwater entering the ocean. Potable water can be drawn from the ocean while still out of sight of the coastline, and the salinity of the ocean is notably lower 500 kilometres (310 mi) out to sea.[citation needed]

There is a natural water union between the Amazon and the Orinoco basins, the so-called Casiquiare canal. Actually the Casiquiare is a river distributary of the upper Orinoco, which flows southward into the Rio Negro, which in turn flows into the Amazon. The Casiquiare is the largest river on the planet that links two major river systems, a so-called bifurcation.