Answer:
Greek city-states primarily used the seas because of the ease of travel and trade.
Step-by-step explanation:
Many areas between the Greek City-states were separated by mountains / hills. This made transportation through land became really hard/expensive for the people who live in Greece.
This is why they primarily used the seas. For the traders , it gave them expense and faster distribution for their consumers in other city-states. It is also a good way for these traders to escape potential bandit/barbarian ambushes that usually hide in mountains areas.