Water As Shielding

It will probably be more cost effective to look at available sources of
raw materials and then pick structural elements based on what you have
instead of what you don't have. If as predicted half of the NEOs are
burnt out comet cores containing large amounts of dirty water then
structural elements become ice with embedded fibers spun from the dirty
part. I am sure that sometime in the distant future somebody will spend
money creating an all sapphire structure for the same reasons that some
people spend money on glitz now.