Re: sort of repeating others but ..
>shorter wavelength than a microwave oven (which is particularly good at being absorbed by water hence its cooking ability),
Something of an urban myth that 2.45 GHz is special for water absorption. It's in a good area of the EM spectrum for the purpose, granted, but it's no better or worse than other frequencies nearby.
"There's nothing special (like resonance), when it comes to absorption of microwaves by water at 2.4 GHz"
"The frequency choice was based on a combination of empirical measurements of heat penetration for various foodstuffs, design considerations for the size of the magnetron, and frequency considerations for any resulting harmonic frequencies." http://electronics.stackexchange.com/questions/125692/why-2-4-ghz-band
I believe early experiments used the band (mainly for the 2nd and 3rd reasons in the paragraph above), then it became a general ISM band afterwards once it was already in use.