. . . unless enough mud accumulated to cover it up
Hence what you said above about any remaining organic material being buried deep in the sediment.
Amanda, Mike, I will also venture to say that, with the Hunley, compactness also made a difference. Consider that those older submarines were tightly constructed to allow just enough space for those on board. The only organism-consuming microbials in salt water to have entered that vessel after it sunk would likely have been miniscule in size and design. In light of this, I would theorize, then, that any human remains aboard the Titanic to still be present in significant portions would be in tight, cramped areas deep within the ship, such as in crawl spaces, and even then not much would be left after all the time that has elapsed.