Yes, waterproof panniers are very easy to clean. I have a set of Ortleib backrollers and they look pretty much like new after 4 years of commuting to work Monday-Friday. I give them a wipe every so often and they're good to go.
BTW, the Ortleib panniers are extremely durable - to the point where I'll have to replace my rear rack long before I have to replace the panniers. I regularly load mine up pretty heavy and I've found that the rack is starting to wear thin on the spots where it makes contact with the panniers. (I use a lightweight aluminium rack from MEC). Riding a bike with heavily loaded panniers significantly alters the way a bicycle handles.
A small trailer (designed to haul kids) as a grocery hauler might be another option for you to consider.