There are varying models on the origins of the flood waters. As with many theories, there are strengths and weaknesses. This link explains the different flood models. Some creationists believe in the Canopy Theory, where a canopy of water surrounded the earth prior to the flood. During the flood, that water fell down as rain. Another possibility is the Hydroplate Model. The following the animation video below provided by Dr. Walt Brown of www.creationscience.com explains the Hydroplate Model.