Water As Shielding

Well, UV is readily absorbed by glass, so sunburn may
not be a problem. With 3 to 5 meter depth, the yellow
thru blue end of the spectrum would still propagate.

I note there really IS such a thing as transparent
aluminum(oxide). We call it sapphire. With no other
elemental inclusions, you should get a very nice
spectrum transfer and LOTS of strength however,I have
no idea what the ionizing radiation resistance would
be. I expect major industrial manufacturing of clear
sapphire is possible, even to large, thick sheets.

Gary 7