In mere words it is impossible to fully do justice to the courage of the sailors and explorers in our history. But I will do the best in my limited ability to do so.

In theory a sailor is merely a man or woman who navigates, controls or mans a ship. They are so much more. A sailor in the times of Columbus and all through history are one of the main reasons we enjoy the world as we do now. It was the sailors and explores who had to face untold perils and dangers in the case of all human advancements.
A good reference would be Lief Ericson. He bravely set out from his homeland of Greenland in search of adventure and in his efforts discovered Vinland, aka North America.
Another outstandingly courageous adventurer in history is Ferdinand Magellan. He was the first person ever to lead an expedition across the Pacific Ocean. It was the first successful attempt to circumnavigate the Earth. Although he died before he could witness proof of his theory it was he and many others like him who cleared for perhaps the most famous voyage of all time, Columbus's legendary venture.
So, with these references of great historical figures it becomes easier to imagine the courage it must have taken for these men to do what they did. Try to imagine a life on the sea with no technology or previous experience to guide you. Sleeping on a tiny bunk with the constant rocking of the waves, small, often times maggot infested, portions of food, the constant threat of a storm, the terrifying unknown alway ahead, and months of not seeing your family.
Imagine abandoning everything you knew and loved and setting out on a seemingly impossible journey, with no guarantee of survival, and indeed a very small chance of survival, and you will began to understand what the sailors and captains of the ships endured.
Compared to the great feats of their forefathers, the explorers of today may seem a little, well, not impressive. But do not let it fool you. The adventurers of our time have just as much courage as those of the past.
Consider the astronauts fearlessly leaving life as they know it to risk their lives to explore for the good of mankind. That is what I consider true courage. Knowing, or their case not knowing, what lies ahead, but pushing forward and doing it anyway. Throwing caution to the wind and doing what no one else will do, whether it's in the name glory, honor, riches, or nobility, they do it with a strong face and an unwavering will that brought the world as we know to where it is.