The correct answer is - Mediterranean Sea.
During the Hellenistic era, the most important and busiest body of water was the Mediterranean Sea. This was due to the location of it, because it was a sea on which shores where the biggest and greatest civilizations at the time.
The civilizations in the Mediterranean region were all using it for multiple purposes, trading, military actions, exploration, fishing, for communication... This led to an extensive usage of the sea from a very early point in the human history, and that has continued over the centuries.
