I haven't heard of this "rule" either, probably because it doesn't seem to be very useful, particularly the one hour til hypothermia bit.

You can become hypothermic in much less time in really cold water - generally less than 45 minutes in water temps ranging in the 30s, although there are exceptions depending on clothing and amount of body fat.

In my own experience, doing underwater archaeology on historic shipwrecks in Lake Superior, wearing well insulated dry suits, the final act before entering the water was to pour boiling water into our gloves, which were then promptly donned as we slipped into the 35ish degree water. The dives were terminated after forty minutes or so because we could no longer manipulate a pencil to sketch or write notes.

Cold water immersion is really serious business...