Re: does uv light pass through water?

This question is a mixture of all sciences I suppose, but I will
attempt to answer it from a physics point of view (since that is the
bit I can be sure of).

The simple answer to your question is actually quite easily found if
you have access to a good library. However, it is not quite so easy of
the absorption spectrum of pure water in the ultra violet region on the
internet. It seems that this information is kept hidden away in
journals. See for example:

T.I.Quickenden and J.A.Irvin. "The ultraviolet absorption spectrum of
liquid water" in Journal of Chemical Physics, Vol.72, No.8, pp 4416
(1980).

and

R.M.Pope and E.S.Fry. "Absorption spectrum (380-700nm) of pure
water. II. Integrating cavity measurements" in Applied Optics, Vol.36,
No.33, pp 8710 (1997).

There is, however, quite a lot of data on the infra-red and microwave
absorption of water which probably stems from interest in atmospheric
sensing and LIDAR (laser RADAR). (Use these as search terms in your
favourite search engine.)

However, one good reference that I did find was the following: 

http://www.lsbu.ac.uk/water/
vibrat.html

Which will also give you a much better idea of why water has the
absorptions that it does. However, that wasn't the question right
now. The data on this page does show, however, that on the red-side of
the UV band the absorption of light is actually much less than in the
visible spectrum. However, it can also be seen then that the
absorption rises sharply as the UV wavelength gets shorter.

What we need now are some points of reference. The following article
gives us a useful definition of the ultraviolet region of the visible
spectrum.
 http:/
/uvb.nrel.colostate.edu/UVB/publications/uvb_primer.pdf

From this we get the following:

UVC 100 to 290nm
UVB 290 to 320nm
UVA 320 to 400nm

Now, if we then refer to (fairly random, but with an amusing name)
tanning site:
 http://www.fabuloustan.com/uv
lite.html

We see that it is the UVA light that is primarily responsible for the
darkening of the pigment in the skin and the UVB is responsible for
the burning effect. However, we are also told that UVB has beneficial
properties like producing vitamin D3 and melanin. Oh, and the UVC
doesn't pass through the ozone layer, but you probably already knew
this since this is why we are so worried about the hole in the ozone
layer.

So now, let us get back to the possibility of getting tanned
underwater. Well, either from the graph at www.lsbu.ac.uk or data from
the two articles that I mentioned above we can see that the light
absorption (often given the label alpha) has the following values
(where the units are "per metre").

Format:
[wavelength] [absorption, alpha] [type of radiation]

From the first article: 

196 1.26   UVC
200 0.324  UVC
290 0.0163 UVC
305 0.0112 UVB
320 0.010  UVB

From the second article:
380 0.011  UVA
400 0.0066 UVA

This is an exponential scale so what this means is that if you
multiply the absorption by the depth of water you are considering and
then then raise the number e to minus the value you just worked out
you will get the fraction of light that will reach you from the
surface. Mathematically:

A = e^(-alpha*d)

where e is 2.71828182845905 and alpha is the absorption factor.

So if you were swimming at a depth of 1m in pure water and wanted a
tan (we can consider the worst case of light with a wavelength of
380nm) there would be an absorption of 1%. So this answers the
question - not much of this light is absorbed.

HOWEVER, there are other things to be taken into account. Firstly, there 
is a reflection loss at the surface of the water (a Fresnel loss). 
However, for a flat surface normal to the incoming light, this is actually 
only about 8%. Water is sadly, rarely flat though so there will be some 
extra scattering and reflection loss.

You may have noticed that I have been very specific talking only about
"pure" water. Unfortunately, few of us get the luxury of swimming in
distilled water. Sea, lake and swimming pool water contains a million
and one other things. Usually in the form of something living
(although hopefully not in the latter!). Dissolved salts, algae, sand, 
mud, insects and a whole heap of other things are likely to be the problem 
more than anything else. This answer is getting less scientific. 

If it were me and I was still worried about the problem I would do the
experiment. The most fun way would be to build an underwater light
sensor with a blue film filter. Preferably using low-voltage
components! Good luck and let me know what you measure if you like
this solution. You could also do the experiment in the lab, but you
would be unlikely to get the same water condition since the water
would be still and particles would have time to settle.

I hope this went someway to answer your question.