In water radio waves are not only attenuated but suffer a severe frequency shift so reception frequency in air is not the same as transmission frequency underwater.
I quite agree with the enormous attenuation, but why should there be a frequency shift? Could you point me at some literature for this effect? The propagation speed will be reduced proportionally to the refractive index of water, but I'd have thought that unless moving at close to the speed of light in the medium, the frequency will be unaltered.
My analogy would be that of light - just shorter em radiation - If I put a light source with a given spectrum - eg a green LED - underwater, and observe it from above water in the air, it's colour ie it's frequency is unchanged.
Allan