Was the world in one piece before the Flood?
By this question I am assuming that you are asking if all the continents were joined prior to the Flood. If you ever poured water over a pile of dirt as a child, you realize that moving water has great power to alter topography. The Bible describes the Flood as one that covered the entire planet's surface for a period of about one year (Genesis 7:11; 8:13). At its peak, it exceeded the highest mountain at that time by a depth of more than twenty-two and a half feet (Genesis 7:20). Such a large quantity of water, both in its rising and falling would have greatly altered the earth.
The water for the Flood came by rain that lasted 40 days (Genesis 7:12) and by underground water supplies bursting forth (Genesis 7:11). In addition, the psalmist tells us, "You covered it with the deep as with a garment; the waters were standing above the mountains. At Your rebuke they fled, at the sound of Your thunder they hurried away. The mountains rose; the valleys sank down to the place which You established for them. You set a boundary that they may not pass over, so that they will not return to cover the earth" (Psalm 104:6-9). To remove the water from the earth, God raised the mountains higher and lowered the valleys in the seas. Hence, we can conclude that the world was flatter before the flood than it is today.
There are scientists who theorize that the Flood could have been created by a sudden shifting of the geological plates, which would have split the land continents. (See "Catastrophic Plate Tectonics: A Global Flood Model of Earth History" at http://www.icr.org/research/as/platetectonics.html for one such proposal.) However, since the Bible does not record the specifics of how the Flood was caused, such theories must be built off the remaining evidence. There is no way to state with certainty that this is the way it happened.