So, literally, it's "the Pacific Sea" and "the Atlantic Sea"? In which occasion do we use "cefnfor"?
Cefnfor is the Welsh word for ocean. The Welsh names for the oceans all use the word môr which means sea.
Just a personal choice really, just as in English a lot of the time - 'I like sailing the oceans', 'I like sailing the seas'.
Cefnfor is ocean, whereas Moroedd is more the plural of môr, therefore 'seas' :)