I think Seo Bro is on the right track,
but I disagree with the confidence of his answer.
I think maybe the earth was once covered in ocean, although it's hard to know at the moment.
The two current candidates for world's earliest continent, Ur and Vaalbara were actually quite small compared to continents now.
I saw a documentary, I forget what it was called, that said that after the Earth's original ocean was formed, there was no land, except some islands, and then a process happened to create granite, and that was the start of continental crust.
You see, nowadays, we have oceanic crust and continental crust.
Oceanic crust is thin, smooth and heavy, and continental crust is thick, and bumpy, and generally light, so it floats on the earth's mantle easily.
So the show said that over time, continental crust has been getting created.
When I saw that, it made Wikipedia make sense. Wikipedia provides a history of the continents, that starts with a few small continents, and ends up with the large continents we live on today.
As for the creation of the original ocean, Seo Bro's quote might be correct that the water came from within the earth. I have, however, heard of another theory, that the water came from icy bodies from outer space, coming and colliding with the Earth in the Late Heavy Bombardment.
I'm sorry I can't provide stronger references than this, but I think if you look up this stuff in more reliable places, they will agree with what I've said here.