Oops - sorry for jumping to conclusions Gaz!
Well if you had got very dehydrated, you probably thought you'd lost more weight than you actually had and taking in all those fluids just got you back to where you should have been? In that case, you might not have put any fat back on.
I remember spending too long in the sun without a drink as a teenager and drinking 4 pints of milk back to back when I got home, and another time pint after pint of (diluted!) Rose's Lime Cordial.
It's actually something to be wary about because of the risk of
hyponaetremia or
water intoxication!