The most unusual thing about the way water changes from one state to another is that its liquid state is denser than its solid state, so increased pressure actually encourages ice to melt. This is unlike most substances, in which increased pressure encourages solidification because the solid state is the most dense. Water also requires a great deal of energy to change its physical state, more than most substances require.
The large amounts of energy required to change the state of water apply to both melting ice and boiling water. As with other pure substances, during a state change pure water does not change temperature, with all added or lost heat energy being taken up or released by the change of state. However, the amount of energy involved in these processes is unusual. Further, changing the temperature of water at all involves an unusual amount of energy, meaning that water has a high specific heat. The specific heat of a substance is the amount of energy it takes to make a given mass of the substance rise a specific amount in temperature.
The lower density of solid water compared to liquid water is very important to life on Earth. Because ice floats in water, it forms an insulating shell over the surface of bodies of water, rather than sinking and making it more likely those bodies would freeze solid.