Of course the other thing to consider is the level of your water table. If you have a very shallow water table, I don't think you'll be able to seal the water out permanently no matter how hard you try. However, if you have some natural slope to work with, a deep water table, lay a well drained gravel bed, and can put in a layered system to keep surface water away, I don't see any reason you'd ever have a problem even in the wettest of years. Personally, I'd use something like a car undercoating, bed liner or a concrete coating (the paint on tar to prevent basements from leaking), then gravel with french drains, a layer of landscaping plastic, a layer of soil that slopes away from the container 1-3' thick, then another layer of plastic before putting your surface soil on.
Even if each layer has some compromise in it, the amount reaching your container wall will be nominal and easily drained away.