2slow2matter-
If you take a look at various barges (both river and ocean going) you would find that they are flatbottomed. This is because this allows for the greatest load carrying per sq ft of deck area. The fore and aft ends are sloped slightly to reduce drag during the tow. I believe we can trust these naval engineers, who have spent years and millions of dollars in research, to determine the greatest LOAD carrying ability. Please notice I have made no comments on stability as these are generally so wide with relation to their height/depth that it does not become a significant factor.
BTW, the Germans wanted to use Helium, and did in their earlier airships, but the US refused to sell it to them. But that is another story also.