There are any number of ways to waterproof the interior. if your really worried about it you could always line the interior with 1/4" plexiglass sheets. My idea was to use pond liner attached to the interior of the container with sillicone.
No..in fact I'd like to thank you for posting that link. The idea stemmed from a design to build an underground house out of shipping containers in a very similar manor although on a much larger scale. In any case there would be left over containers and that was what started me thinking about this. A large part of the idea is to end up with a monster tank that rivals built-ins but is in fact movable. (not by 4 drunken friends and a pick-up truck but movable just the same.)