It's the water vapor above the sea water that is sucked up by the hurricane that releases heat when it condenses high in the sky. So, the power of the hurricane is related to the evaporation rate of the water.
I've read that strong hurricanes transform the water surface into a big volume of small water bubbles. As the winds become stronger and stronger, you get more and more waves that start to break. Above a certain wind speed, there us no clearly definable water surface anymore.
The total evaporation rate is proportional the the water surface. In case of strong hurricanes, the water surface is the sum of the surfaces of all the water bubbles which is huge...