Is being underwater nature's low-pass filter, sort of like the natural audio equivalent of visual blurring?

I'm going to take a guess at the observations that have lead to your questions.

Above water sounds are muffled when one is underwater. This is due to an impedance mismatch between two mediums, air and water. When airborne sound encounters the surface of the water, only a small fraction of that sound continues to travel through the water, the rest is reflected off the water's surface.

Similarly, our ears and vocal cords have evolved to be efficient transducers above water, but not when submerged. On the other hand, whales have evolved very efficient underwater transducers and can communicate with other whales over 100s, maybe 1000s of miles/kilometers. They typically communicate at low frequencies since attenuation increases with increasing frequency. This is similar to what happens in air, like when comparing thunder from a distant lightning strike to a strike that happens nearby: the distant strike sounds like a deep rumble since the mids and highs have been absorbed by barriers and the air while the nearby strike results in much more high frequency "crack" and "sizzle" sounds.

Just riffing over here. Sound is rad.