I stand corrected - here's
a page that agrees with SusanB - well, it suggests 64 oz of water a day. It does mention overhydrating - "Higgins said it is possible to drink too much water, which leads to psychogenic polydypsia, or water intoxication."
This page, however, bumps it up to a gallon a day for body builders.
I was thinking that SusanB's 3 litres might possibly be a little low for someone who is exercising....