Okay, I think I've found the answer.
The Teivah was definitively not a boat shape according to the Ibn Ezra, Abarbanel and Rav S. R. Hirsch.
Ibn Ezra:תיבה ולא ספינה כי איננה על צורת אניה ואין לה משוטים - a box, not a ship as it was not shaped like a boat and had no oars.
Abarbanel: והנה צוה אותו בבנין תיבה לא ספינה ולא אניה לפי שלא היתה כדמות ספינה עם תורן ונס ושוטין אבל כדמות תיבה ולא היתה מרובעת אבל היתה כדמות משולש וראשו חד כדי שלא תתהפך מהמים ולא מהרוח - and behold He commanded him with the building of a box, not a boat or a ship because it should not have the form of a boat with a mast and a flagpole and oars, but in the shape of a box, and it was not square but rather triangular and it's top was sharp (pointed?) inorder that it should not be overturned by the waters nor by the wind.
Rav Hirsch:תבה (a receptacle or vessel) is mentioned only in this story and that of the rescue of the infant Moishe. In both cases, a Teivah helps a human being survive in the water by keeping him afloat. The origin of the term is obscure…In any event the Teivah is shaped like a box or chest unlike a boat. It is wide at the bottom tapering toward the top, the reverse of the shape of a boat; for it is not designed to cut through water, but to be borne on the surface of the water.
As of yet, I have not found any peirush that says otherwise.
So the guys who think that the picture above is the actual fossilised remains of the Teivah can go and fly kites for the moment.