Water on top of the surface freezes as it is in contact with cold air outside. The ice layer being less denser than water, floats on water. So it forms a protective layer above water.
Ice has a poor conductivity of heat as compared to water and hence, the cold air outside has less impact on the water underneath the ice. Water freezes slowly from the top. Also, water has a high latent heat, so to become ice , it takes longer duration.