Microwave and 2.4GHz
"if you try to cook your pizza with radiation at 900 MHz, you'll have to add a radiator. You need 2.4 GHz to boil water; that's the frequency where water is most effective at blocking the waves"
For some reason, a lot of people believe that. Not true, industrial microwave ovens often use 915MHz. There is nothing magical about water and 2.45GHz, in fact if you use WiFi outdoors you will hardly notice a drop in signal strength due to rain.
The first big absorption band of water is 22.2GHz.