Nothing, the temperature remains the same. As the water boils, the liquid water is converted into steam. All of the heat being added to the water is used to change the water into steam and the temperature remains the same (100 degrees C). The term for the amount of heat needed is latent heat of vaporization and is 540 cal/g/deg C for water. The same thing happens when ice melts, the temp stays at 0 deg C while the solid ice is converted into liquid water btw. Also Boyle's law is a gas law and does not really apply here. It relates to an ideal gas only and states that Pressure times Volume is constant, so as either changes the other will change.