Here's a page with a cross-sectional drawing of the Ark:
http://www.6000years.org/frame.php?page=noahs_ark
It's pretty much as described above - kind-of oil-tanker-shaped.
The site also has the coordinates to a site of an Ark-shaped feature on top of an ancient mud-slide. (Ark remains or geological formation).
In my opinion, a Dutch sailing klipper is better than a Thames barge in open water, but neither is really suitable for ocean work. Even for the short trip from the Amstel (Netherlands) to the Thames, you would not risk it in anything except perfect conditions.
All barges have the same problems of being overly-stable, so 'snapping' quickly back to horizontal from a roll - very tiring for the crew and the hull. They have little freeboard and can easily take on water. The shallow flat-bottomed hulls and can't 'grip' the water like a keeled boat will. Around estuaries, with the lee-boards up they're even more of a pain to steer - they skate sideways around curves and follow the currents.