The biggest problem with cargo submarines would be access - i.e hatches. Anything other than a tanker needs hatches. They are basically big holes in the ship's structure, and cause problems with both structural strength and watertight integrity.
But the majority of cargoes need the hatches to be big, just to get the stuff in and out (look at any general cargo boat). The exceptions would be specialised stuff, or high value, low volume items - i.e. drugs.
The broblems could be overcome - but would the benefits outweigh the costs? Why do you want your cargos shipped underwater? OK the boat can't bee seen - usually - but there is not exactly a shortage of platforms hunting for submarines in a war environment...
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