Ice floats on water because it's less dense. The same mass of water has more volume in the frozen state than in the liquid state. Density = mass/volume, so increasing the volume while keeping the mass the same results in a lower density.
Water is one of only a few substances that expands when it freezes. It's called the density anomaly. It occurs because of the hydrogen bonds, which are strong intermolecular attractions, between water molecules. These attractions cause water molecules to come together in a very open crystal structure as they begin to freeze, causing an overall increase in volume.
Because the water molecules are farther apart in the solid state than in the liquid state exerting pressure on ice will cause the ice to melt as molecules are pushed closer together. This helps makes ice slick. As an ice skate glides over ice the pressure temporarily creates some liquid on which the skate glides.