Who is responsible for the ocean?

Answer 1

God.

Or if you're a more science-y person, when the Earth was a big molten ball it gradually cooled down on the outside, but heat continued to be released from volcanoes. Water trapped inside the volcanic materials was also released into the atmosphere in the form of water vapour. As it condensed, it fell down as precipitation. Since gravity causes water to flow downhill, the vast amounts of water that fell began collecting in the lowest parts of Earth's surface, the deep basins which are now oceans.

Some scientists also think that much of the original water on Earth came from the ice in comets that hit the young Earth.