We can’t see through walls because electrons in the structure absorb visible light. Some light reflects off the wall, but no light gets through. Radio waves – the same stuff as visible light with a longer wavelength – go straight through. This is because electrons in a wall don’t absorb or reflect radio waves, due to their shape. Electrons in water act in a similar way to visible light so they don’t absorb or reflect most of the light. Instead they allow it to pass through relatively unimpeded, absorbing wavelengths like infrared and reflecting invisible UV.
Answered by Rik Sargent