In addition, "the South" did not have the ships nor the facilities to support the ships. They didn't have the trained crews and officers to man the ships. They didn't have the network of commercial contacts across the business world to operate with, and they did not have the year-round ports/markets to support a real shipping industry (maybe New Orleans because of the control of the Mississippi basin). It is much, much easier to run an import/export business from Boston, New York, Philadelphia or Baltimore than it is from anyplace in the Deep South.
New York city had an intimate relationship with the slave cotton producers and altho you are right that southern ports did not have the "contacts" nor "shipping lanes", New York ships would leave the New York harbors and stop at southern ports to load up on southern exports and by extension the south had all of those available to them...New Orleans only had those lanes developed around 1850...Baltimore only exported Baltimorians and they didnt want to swing by savannah on their way to Europe...so Baltimore failed over time.
Hmm. I think you have not taken the time to digest what I said -- or possibly I did not explain it well enough.
Generally speaking, there had been a race between New York, Boston, Philadelphia and Baltimore to become the dominant seaport and market of the US in the early nineteenth century. The key to winning that race turned out to be access to the interior of the country, the other side of the Alleghany Mountains.
Boston, Philadelphia and Baltimore were handicapped by geography. New York won because they had the most practical route and built the Erie Canal (opened in 1821) to connect the Great Lakes to the Hudson River and New York City. New York also had the most practical RR route (the collection of RRs that eventually became the New York Central system). Boston was handicapped because any route they could have ran through New York State. Baltimore and Philadelphia tried going up the Susquehanna, but couldn't get above Harrisburg; mountains blocked canal routes. The Pennsylvania RR's were 2nd to reach Ohio; the Baltimore & Ohio finally reached the Ohio River valley around 1850 or 1851. For about 25-30 years, New York had vastly superior access to the West and the best harbor on the East Coast. No other port could match it (and New Orleans lost a lot of trade to the East Coast as a result).
New York became the hub for shipping in the US. By 1860, almost 2/3rds of the tariff revenue collected in the US is collected in New York harbor alone. There are a lot of reasons for that, including the incomparable connections to the rest of the country, but also including the vibrant commercial environment (banking, trading, etc.)
Ships from Baltimore would go anywhere they could make a buck -- but for most of the year that did not mean sleepy Southern ports like Savannah and Mobile. The port would be crowded when the cotton crop was in -- but what reason was there to go there when the cotton crop was not? A ship makes money best when it carries goods
both ways; what is the return cargo from Savannah or Mobile if it isn't cotton?
If a ship wants to transport goods from Europe to the US, most of the time the best market is New York. Go there, sell your cargo, pick up another and go back. The journey is shorter. There are more buyers and sellers in New York. It is easier to pick up cargo so you don't have to delay (time is money) or voyage empty (gack!)
Philadelphia and Baltimore survived and prospered (particularly when their RRs reached the West), but they could never make up the 25-30 year head-start that the Erie Canal gave New York. Meanwhile, prices for goods dropped in the Old Northwest as soon as the Erie Canal opened; trade exploded; immigrants rushed in. New Orleans suddenly had a great deal of competition for the trade of Ohio, Indiana, Illinois and the rest.
There is a tremendous amount of infrastructure and development needed to create the type of success New York had. "The South" generally did not want to make the type of investment (in people and infrastructure and education) that required. They wanted to exist as a type of planter society based on agriculture with little emphasis on education, a tiny middle class and a sneering attitude about northern "greasy mechanics" and "shopkeepers". Unfortunately, you need a large group of educated people with experience in arcane skills, supported by many "greasy mechanics" and "shopkeepers" to build the kind of commercial businesses they did not have.
It is one thing for individuals to overcome problems through talent and skill and great effort. It is a vastly different thing for large groups to do the same. So you might find an individual planter in "the South" who would be sharp enough and determined enough to become a big international trader -- but there'd only be a few of them, and it would be much easier for a planter to just keep doing what he was already doing than to do all that was needed to get into the shipping and trading business.