How can you infer that density decreases when ice melts from the water phase diagram?
Draw an isotherm at a low enough temperature (&lt; the "triple point temperature of 0.01°C), so that it passes through "all three" phases (ignoring the high-pressure ices that are not relevant to the question).

Note that as pressure goes up you go from solid to liquid.  Increasing pressure always favors a denser phase, so liquid is denser than solid for water.

Water is not quite unique in this respect. Silicon and germanium are two elements that do the same thing.