Well it was radar. The first Raytheon microwaves were really pushing for 3GHz not 2.4GHz. If you like to play Connections, the reason for that is the first mass produced magnetrons were made by gun manufacturers like Colt and Smith & Wesson and the tooling for gun bore holes and magnetron cavities lined up at 3GHz.

The official FCC minutes from 1945 [1] indicate that publicly they were marketed for heat therapy massages not food, with a weird wink, wink that if they could get a carve out for using for medical reasons they could also sell it to the Navy for reheating food as well.

The ISM carve out came after by a couple of years in 1947 because Raytheon had got an exception for this machine, not the other way around.

The whole origin story of why this particular slot of spectrum is full of carts before horses. That water oscillation thing is a common misconception - water oscillates at much higher frequencies [2].

[1] https://www.worldradiohistory.com/Archive-FCC/FCC-Annual-Rep...

[2] https://www.sciencedirect.com/science/article/abs/pii/S01691...