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More precisely: assume one measures X mW/cm² for wavelength Y nm at the water level 0 (sea level). What's the mW/cm² for the same wavelength Y nm at water level Z? I'm most interested in UV-A and UV-B rays.
I assume it partly depends on the water, but I am looking for some estimate.
It turns out that this is much much more complicated than I would have thought, not being a spectroscopist other than in general use of various automated spectrometers for measurement of biological variables. There are a huge number of variables involved and it all depends on illumination sources and the quality and types of optics used to obtain the results, but I think an answer is approachable in general.
So, for pure water the spectrum for absorbance between 250 nm and 550 nm looks like:
Where the dots are absorbances at 10 nm spacing. You can see that the absorbance is has a low at approximately 350 nm and is fairly high at 250 nm, but absorbances below 250 nm are not shown. This is because the materials used to make to optics and cuvettes for measurement in these ranges strongly absorb too. However, I would expect that there is similar variability in the absorbances.
Addition of particulates and other chemical contaminants strongly change the spectra, depending on exactly what the contaminants are and how much is there. However, the same article has another figure that shows a few different measures of water, including "pure seawater" (dark blue, labeled "Lee et al") for the range 350-450 nm, which is only a small part of the UV spectrum:
I also found an article that is available as PDF only, and probably paywalled, that aimed to study the "clearest natural waters", by which they mean clearest seawaters found in the South-Pacific gyre.
I won't reproduce the figures here as the absorbances were measured in the 300 nm - 800 nm range, but suffice to say that the absorbance patterns are very similar to those shown here.
References:
