Answers by people who understand this better than me here [1], but 2.4GHz is not "the" resonant frequency, water has a number of frequencies it interacts with radiation. And water isn't going to behave like a tuning fork anyway, it would be more analogous to sand. Microwaves interact with water by dipole interaction, and they just unload heat.

That's why wifi isn't going to do much to your body, any cooking of you being done by radiation is trivial compared to your own body heat.

[1] https://physics.stackexchange.com/questions/169173/what-is-t...