Every full size submarine ( diesel /AIP/ or nuclear) has had the same overall approximate height from the top of the periscope to the keel since WWII…. They all end up being between 50 and 65 ft except for the SSBN. None of those can operate anywhere in the Mississippi’s shallow delta, let alone the actual river. Magnolia Anchorage, for example, is 65 ft at the deepest and that is only half way to New Orleans.

For a sub to operate, they need at least a couple hundred feet under the keel and about a quarter mile to turn around.

There is only one attack submarine that i am aware of that could make it past the delta. Although it would be extremely hard on the 8 man crew. With a with a handd crank powered propulsion top speed of around 4 mph 40 ft langth and a beem of only 3.5 ft it could operate quite far up the Mississippi although it would have to put to shore every night.

The H.L. HUNLEY

Launched in July of 1863 given over to the Confederate States to operate against US blockade it was a ship designed with an a spear explosive that was to ram an explosive into the wooden holes of US ships. Then it was to back away a good distance where the explosive would run out of its safety line when that happened the explosive was to trigger. It was successfull only once on the 17 of February 1864 Against the USS Housatonic unfortunately due to a malfunction of the spear explosives trigger it was set off too early causing the demise of both ships. This was unknown until 1995 when an extensive surve of the Housatonics resting place.

The Hunley was raised in 2000 and is now on display preserved in a tank of water at the Warren Lasch Conservation Center in Charleston South Carolina.