It's believed that the early oceans were formed by water vapor that originated from volcanos.
Water vapor raised from the volcanos into the air.
As the water vapor cooled, clouds in the atmosphere formed.
Eventually it rained causing flooding, and designing, creating the large bodies of water we know today.
Even though the ocean doesn't look like what it did millions of years ago, it's still the same water
That being said, water vapor from volcanos is the reason why we have oceans.
