The pure rotation spectrum of water vapor extends into the microwave region.
Liquid water has a broad absorption spectrum in the microwave region, which has been explained in terms of changes in the hydrogen bond network giving rise to a broad, featureless, microwave spectrum. The absorption (equivalent to dielectric loss) is used in microwave ovens to heat food that contains water molecules. A frequency of 2.45 GHz, wavelength 122 mm, is commonly used.
Radiocommunication at GHz frequencies is very difficult in fresh waters and even more so in salt waters.