There WAS one ocean, once, where all the waters had been collected together around Pangaea.
The formation of supercontinents happened more than once, and Pangea was not the first supercontinent to form as far as we know. Rodinia is the first so-called supercontinent that we have evidence for. While we have evidence that they existed and a general knowledge of how the plates fit together, it is far from complete and Pangea, at least, had a bounded ocean in between the land forms.
By the way, Pangea won't be the last and some models suggest the formation of a new supercontinent some time in the deep future.
So if we were to believe your version of things, Pangea formed on the third day and Rodinia on the first? Does the bible talk of a third iteration of supercontinent?