Don't take this as gospel either
I seem to remember this being featured on a show Robert Llewelyn did with the OU about proving / disproving bits of films using science.
I think it's something to do with water being a really poor conductor of electricity and it not conducting electricity evenly unless it's perfectly pure.

No doubt someone who knows what they're talking about will be along in a minute to explain properly.
(Hagis, Tue 5 Oct 2010, 23:43, 1 reply)

I might be lying
but I'm 99% sure that absolutely pure water does not conduct electricity, it needs an electrolyte such as any of the shit that's within the pipes and anything that's come off your skin.
( sandettie light vessel automatic New Twitter - @bollocksreally, Tue 5 Oct 2010, 23:47, closed)

It's very possible I remembered it wrongly.
Though I think it's quite certain that the well-meaning but ultimately half-arsed advice we're dishing out about toasters, water and baths shouldn't be used as fact.

EDIT:

Pure water won't, water that has salt in it will conduct electricity but it's not the salt that conducts. Or something.
www.madsci.org/posts/archives/2001-12/1008279728.Ot.r.html
(Hagis, Tue 5 Oct 2010, 23:52, closed)