Take a look at this document talking about lightning myths - http://newweb.wrh.noaa.gov/hnx/LightningMyths-1.pdf
You must avoid any conducting path
leading outside, such as corded
telephones, electrical appliances,
wires, TV cables, plumbing (including
plastic pipes with water in them),
metal doors or window frames, etc.
Also from here: http://indianapublicmedia.org/amomentofscience/can-lightning-strike-you-in-the-shower/
Metal is a good conductor, so if
lightning strikes an unprotected
house, chances are the current will
travel through any metal pipes. What’s
more, the tap water moving in these
pipes contains impurities that help
the water conduct electrical current.
When you’re wet, the natural
resistance of your body is cut by
half, and the salt and contaminants on
your skin decrease your resistance
further. So basically, being wet can
make the difference between an
unpleasant electrical shock and a
deadly one.