There’s several ISM bands, though, pretty evenly spaced. The 13.5MHz one is used for passive RF chips like on credit cards, for example. They’re skinny, but for purposes where bandwidth doesn’t matter they can be. For other purposes bandwidth is scarce enough there has to be tight regulation.

Actually high water absorption happens in mm wave bands up in the hundreds of GHz (and THz too, if we could make a decent transmitter). Those fucked up riot control devices that make your skin feel on fire work based on that principle, because the heat will only go deep enough to hit pain receptors. Presumably, they stop working if you get a water mixture of any kind on the window, too.