But unlike many boating questions, this one has a refreshingly straightforward answer: It's not about size. The U.S. Coast Guard doesn't require a captain's license based on the size of the boat, but based on what the boat is used for.
No, you are no required to hire a captain if the vessel is registered as a pleasure craft. That said after about 40 feet you will most Likely need crew to safely run it.
Virtually all commercial ships have captains who are credentialed by their nation's maritime agencies.