The USB Drive type used with PromoKeychain waterproof USB Flash Drives are 100% waterproof. The entire circuitry and contacts are encased in a waterproof resin.
I have gone surfing in the Pacific Ocean with the USB KEY model (http://www.promokeychain.com/custom-usb-flash-drive-metal-usb-key-p-169.html), which uses the same memory chips as other PromoKeychain waterproof USB Drives. No water damage has ever occurred, and the USB contacts have shown superior resistance to the salt water.