What is the theoretical visibility limit through pure water?

Question

In diving we rarely get vis better than 20m. What could be the max vis achievable in a low airrated body of pure (or close to pure) water?

Answer 1

The absorption coefficient of pure water is approx 0.00025 per cm. (Actually, its a function of light wavelength and water temperature).
i.e., log to the base 10 of (initial intensity/final intensity) is 0.00025 per cm.

Thus, 90% of the light is lost in 40 m and 99% of the light is lost in 80 m. Ofcourse, if you are short sighted (like me), you lose visibility much earlier!

Answer 2

Much much more than 20 m (;-))
Based on references below we can see that some light frequency Will travel further in water then others.

I=I' 10 ^(-αd) (see ref 4)
I - intensity at d distance
I' - intensity at 0 distance
d - distance
α - absorption coefficient (25°C were 0.0005, 0.0013, 0.003 for wave-lengths 2050, 1950 and 1900 Angstrom)

or
log(I)= - αd log(I')
d=log(I'/I)/α (in cm)
Reduction in intensity by 50 % we have
d=log(.5)/0.0013 (for 1950 A wavelength)
d=231 cm or 2.31m
d=log(.5)/0.0003(for 1900 A wavelength)
d=1003 cm or 10 m