There was a molten hell hurtling through space, but over time the surface cooled into a very thin shell. On it, puddles of water collected and the inhabitants of the shell called them the oceans.
This is the cross sectional view of the Mariana Trench:
And this is a cross sectional view of the Atlantic Ocean:
And this wonderful illustration compares the volumes of total water (left) and total atmosphere (right) relative to the volume of the earth:
Typing and design by Gib Hemani