I have cities, but no houses; forests, but no trees; and water, but no fish. What am I?

I am located in a harbor, but I am not in a ship. People visit me for trade.

I look like a mouth, but I am not a body part. People often dive here.

Immortals are the land, and those who inhabit it, but they won't grant eternal life, for our fate is a gift that will follow us even beyond the circles of the world.

Sometimes narrow, sometimes wide. Shine or rain, outside I remain. Heat or snow, from place to place I will still lead you to and fro