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Many people believe that Earth was one big ocean at some point in its past.
During Noah’s time the earth was completely covered with water.
There are many other examples, not all related to Noah. See here, here, here, here and so on.
Is that true?
I'd also be interested to know when this was the case, or what was the largest percentage of the Earth that was ocean at any one point.
I could not find any scientific evidence that the earth was once completely covered by water (not lava).
However, Australian scientists have hypothesized that the land share on a planet 2.5 billion years ago was only 2-3%. If it is true, then the largest percentage of the Earth that was ocean is 98%.
Also:
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.
At the same time, the oldest known rocks on Earth are almost exactly 4 billion years old.
According to Essentials of Oceanography (2012) at page 13
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 authors of A case for late-Archaean continental emergence from thermal evolution models and hypsometry Earth and Planetary Science Letters volume 275, pages 326-336 conducted 100,000 simulations to determine the fraction of the Earth covered with water 2.5 billion years ago.
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.
