For a long period, Greeks, Phoenicians, and Etruscans frequented, at the same time, the Mediterranean Sea. This forced their seafaring to behave, at the same time, as pirates at sea, to break down competition, and as traders in the emporia, to sell their goods. Navigation was based on the experience of sailors since there were no instruments and methods to estimate sailing parameters. The empirical knowledge of seafarers was based on the observation of environmental and astronomical daymarks, and on their ability to perceive signals from the atmosphere, and from sea animals. Navigation was mainly carried out at such a distance from the coast as not to lose sight of it; but in the Mediterranean Sea the coasts, often mountainous, allowed us to have in sight elevated points of reference, which also allowed offshore navigation. Merchant vessels were not as fast as combat, but they could carry large loads of goods to be sold in any emporia. The average speed of a ship was around 5-6 knots, therefore the longest voyages could require considering stops for rest and bad weather. In this case the journey required a winter stopover, and the navigation continued into the following season. Therefore, we understand the importance of “weather forecasting” which could be done with a careful analysis of the warning signs that came from the sky, from the flight of birds and the behavior of fish and other aquatic animals.