Why did the navy and merchant ships sail over the world's seas during the late Middle Ages?

What does the quote describe? “the closeness of the place, and the heat of the climate, added to the number in the ship, which was so crowded that each had scarcely room to turn himself, almost suffocated us….the shrieks of the women, and the groans of the dying, rendered the whole a scene of horror almost inconceivable.”