That our earth has never been completely covered with water since land masses first arose from its primordial global sea has been firmly established by modern science in more ways than I can possibly here begin to mention. For a discussion of this subject matter see Problems with a Global Flood at http://www.talkorigins.org/faqs/faq-noahs-ark.html .
Conservative "fundamentalist" Christians' typical response to such information is to say that they choose to believe the Word of God over the findings of scientists. This certainly sounds quite noble. And I suppose I would commend them for their stance if such a stance was called for by the Bible itself. But it is not. For a careful study of the flood account in Genesis reveals that the Bible does not tell us that the flood of Noah's day was global. And an examination of the scientific "evidence" presented by Christian fundamentalists in support of a global flood, sea shells on mountain tops and the like, quickly reveals that the presenters of this so-called evidence have a very poor understanding of science. ( By the way, sea shells on mountain tops are the result of earth's plate tectonics causing land masses to slowly rise from the sea over many millions of years. This process is an ongoing occurrence and can be proven by comparing the measured heights of various mountain peaks today to their measured heights just a few years ago.)