According to the modern scientific views, the ocean appeared 3.5
billion years ago as a result of magma degassing and subsequent
condensation of atmospheric vapor. Most modern ocean basins emerged in
the last 250 million years as a result of the splitting of the ancient
supercontinent and the cleavage (spreading) of lithospheric plates.
An exception is the Pacific Ocean, representing a shrinking of the
ancient Panthalassa ocean remnant.
The physical expanse and distribution of the early ocean is a matter of some controversy. Most researchers hold that masses of rock have always protruded through the ocean surface to form continents. However, some recent studies suggest that water may have covered Earth's entire surface for some 200 million years before continents emerged.
The input parameters are described by Gaussian distributions using the mean values and standard deviations listed in Table 1. From [100,000] realisations, we predict a distribution of the area of emerged land and of sea level.
The Earth was completely flooded in 9322 of the trials
The medians for [area not flooded] range between 1.79 and 2.84% of the Earth's surface.
In conclusion, the research found a 9% chance that the Earth was completely covered by water, and the best estimate was the Earth was between 97.16 and 98.21% covered by water.
Yes, the Earth was entirely ocean before any continents existed.
The continental crust started forming 3.7 billion years ago. The first "micro continents", even if they were tall enough to have dry land, would consist of islands - not a sizable barrier to make you consider dividing the ocean into different places around it. That is, even if you had some dots of land, you would not consider different parts to be different oceans.
The link above mentions the oceanic vs continental crust — but was there water covering it? Look up some dates for that:
The water to fill the ocean was present at the latest by 4.1 billion years ago. So, besides being covered with oceanic crust, the Earth was indeed filled with water at that time.
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.