Earlier, I posted the following chart on another thread, represents the total internal energy of water. At 370 C, water comes a gas (single molecule).
I will try to explain this in more details: Below 370C, water exists as vapor. Vapor is of just cluster of molecules stack against each other. The gaps between the molecules increase with temperature, causing overall volume increase.
The study of gradual phase change phenomenon is called enthalpy. Simple put, water molecules are held together by an internal energy called Van der waal force. This force weakens when external energy is applied, the gag between each molecule increases and eventually reach total separation, the single molecule become gas. At this time, the Boyles law applies.
You can look this up on the steam table or enthalpy property of water in any reference book.
Since pv = constant. Keeping pressure constant means change of volume is proportional to temperature change. You can invert the above chart and you get your answer there.