Christians don't say that.
There is not enough water to cover the entire earth NOW.
But remember, Noah's flood occurred before the earth was divided. There was only one continent, and that continent did not have mountains like we have today.
During the flood, God tore open the "firmament," both below the ocean, and above the stratosphere. And subsequent to the flood, about 95 years later during the time of Peleg, God separated the land into the continents we have today. And the force needed to move these continents caused some earth crusts to break apart, and they started to go up at an angle. Mountain ranges were formed. This process included many earth quakes and volcanoes.
In summary, before the flood, there was enough water to cover the entire planet after God opened up the firmaments above and the firmaments below ground.
But now the mountains are too high.