Water doesn't resonate at 2.45GHz, it's a common myth.
The truth is that liquid water doesn't have a resonant frequency as the molecules are all hydrogen bonded togeather.
A good 2.45GHz squarewave will have strong harmonics up to 24.5GHz and still be quite powerful at 122.5GHz which are no legal, in fact half of the power will be in harmonics.
To generate a good square wave the transistor will need a GBWP of 100 times the frequency - you'll need to have switching times of 40ps.
To get this sort of speed at this kind of power level, things get really expensive, you need all sorts of exotic components like TWT amplifiers and klystrons.
I say scrap the idea, it probably won't work even if you had the know how to make such a device it doesn't really have any other applications other than microwave transmission.