A review of the history of microwave cooking implies that the interaction of 10 GHz microwave radiation with water was understood by WWII.
"effective penetration and heating favors wavelengths in the cm range (1–30 GHz). Higher frequencies cannot penetrate sufficiently, and much lower frequencies require very high fields, and can have poor coupling unless
making use of direct contact or inductive heating. In the years following WWII, emphasis was thus placed on frequencies in the low GHz region, where both the historic development of microwave power sources and serendipity converged."
"effective penetration and heating favors wavelengths in the cm range (1–30 GHz). Higher frequencies cannot penetrate sufficiently, and much lower frequencies require very high fields, and can have poor coupling unless making use of direct contact or inductive heating. In the years following WWII, emphasis was thus placed on frequencies in the low GHz region, where both the historic development of microwave power sources and serendipity converged."
https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9393469