Even many regular PC coolers technically take advantage of boiling water, in that they have heat pipes that rely on water evaporating at one end of the pipe and condensing at the other in order to maximise heat transfer to the heatsink fins.
Really bad idea, you will need to constantly fill the tank, and you will also need to make sure system is mounted only one way or water will totally miss the heatsink. It's like water cooling without the enclosure.
It will evaporate in less than a few seconds also.
There is a simple system to meter water so it only goes out as fast as it can evaporate, it is called a wick.
As pointed out above, heat pipes operate on a similar principle, albeit in a closed loop: the hot end in contact with the CPU, a cool end in the fin stack, and wick material on the walls to move condensed liquid from the cold end back to the hot end through capillary action. Passive closed-loop phase-change cooling in a tube.