I've joked that you could cross the Sahara with Ortleib panniers filled with water. You would need to beef up the mounting hardware, the racks and the mounting points. Come up with an appropriate closure at the pannier top. But those panniers are totally up to the required strength and waterproof- ness. (I use mine for shopping at the farmer's market for produce. When needed, I wash them like flexible pots. Partially fill with soapy water, scrub, rinse and hang upside down to dry.)

Edit: after reading andrewclaus' post - I'll pass on the experiment. An angry camel could kick my pannier and it if didn't repent and carry me to water, that would be it.