"Opaque" has no absolute meaning.
Aluminium is essentially opaque with less than 100nm. Silicon is a bit translucent at 1µm. Glass get opaque around 0.5m, when you look through the width of a window. The Ocean gets dark around -20m.
It also depends on what attenuation you accept or desire and whether the wavelength fits the material. Submarines are said to be detectable by optical apparatus to -30m or -50m, making the Barents, Baltic, Azov seas and the English Channel deadly traps for them.