I'd actually disagree with that statement. When you go fast, you risk pushing a LOT of water under your fenders, and you'll get more water in your intake, saturating the element, as well as water everywhere else.obsidian said:
By going fast you risk splashing water into your engine compartment, and depending on how cold the water is, and how much gets up there, you can crack your engine block (the sudden cooling of the metal). You also risk losing control, soaking your brakes enough that their power is greatly diminished, soaking your engine electrical system (read: stalls your engine) and then getting stuck.
If the water is lower than your intake, go nice and slow, so as not to splash. If the water level is higher than your intake, go another direction!