If you're looking for a usb one you're looking for a webcam, notta backup cam. Backup cams usually a composite output. I'm not aware of any USB cams that would be waterproof. I just bought a cheapie off ebay a few years back and its been working like a charm ever since. Check out the mp3car store for some selections too.
If you really want to have it be USB connectible simply buy a USB capture device and send the backup camera sensor through that. Thats how i do it.