The sea was literally at the center of much of the ancient world. The answer to this question could branch out in many different directions. The Phoenicians were the ultimate sailors of the Mediterranean world, engaging in trade across the sea, designing warships, and building trading outposts where their ships could land. They even acted as trading agents for other peoples, carrying the cloth of Mesopotamia, spices of Arabia, and gold, ivory, and slaves of Africa. The Phoenicians even spread their writing system across the sea along with the very first alphabet.
Another possible discussion point for this assignment would be a look at the centrality of the sea for the ancient Germanic peoples. The Vikings, for instance, set out on the high seas to raid and trade with neighboring peoples (and peoples a bit farther away). They also used the sea as a route for exploration, reaching all the way to Iceland, Greenland, and North America. They drew food and trade goods (like seal fur) from the sea as well, and they used the sea to cross over to England where they raided and then settled, pushing previous raiders and settlers (the Angles, Saxons, and Jutes, who had arrived from over the sea a few centuries before) out of the way. The languages of these groups also spread over the seas, for the Angles, Saxons, and Jutes first brought their ancient Germanic language to England, a language that would become Old English and eventually modern English.
An answer to this question might also include references to ancient naval battles like the Battle of Salamis between the Greeks and the Persians in 480 BC, the Battle of the Delta between the Egyptians and a mysterious sea culture about 1176 BC, the Battle of the Aegates Islands between Rome and Carthage in 241 BC, and the Battle of Arginusae between Athens and Sparta in 406 BC. These battles make for fascinating stories as well as examples of the centrality of the sea in ancient culture.