The difference between an ipod or phone and a usb pen drive is the ipod/phone has a battery, and the moisure can and will cause conducting paths where not wanted = game over (phone if your quick you can get the battery out before water does its damage).
A usb pen drive is powered through the USB port (some have small batteries for other purposes) so in most cases its not powered at all while wet, as long as its dried out beforehand should be fine, plugging in when moisture is present will most likely wreck it.
Some usb pen drives are near indestrictable, under normal mishap these days.