Yea, as far as I know it varies state by state. I know growing up in New York State, an operating (boating) license was needed on top of the registration for motorized watercraft until the age of 18, at which point a driver's license + age = de facto boating license. Passengers or other factors did not matter, just as they do not while driving a car. To be honest, I don't recall if any sort of license was needed for sailing or other non-motorized craft, but I'm thinking not. Apparently nowadays in NY licenses are also required for jetski operators regardless of age, but for boats the old rules still apply.
If you want to document your boat with the U.S. Coast Guard, she must measure at least 5 tons net. For practical purposes regarding pleasure boats, net tonnage is understood to be 9/10 of gross tonnage.To calculate gross tonnage, take half of the overall length L times overall breadth B times depth D (the internal measurement of the hull, not the draft), all in feet, divided by 100. In other words:˝ (L x B x D ÷ 100).
Roughly speaking, 5 net tons corresponds to a moderate-displacement boat about 30 feet in length. A heavy-displacement, long-keeled cruising boat could be as little as 25 feet overall but still have sufficient volume below decks to qualify for federal documentation. A documented vessel must be owned by a U.S. citizen. The captain (and any other officer) must be a U.S. citizen as well, although crew members need not be. Documentation gives you the legal right to fly the special U.S. Yacht Ensign (in home waters only), an authority that is not officially granted to other yachts— although many wear it anyway.A documented yacht’s title is also proof of ownership because it records the liens, mortgages, and other financial liabilities (if any) that she carries.
If you are planning on becoming captain of your own boat and using it for charter operations, you will need to be licensed by the United States Coast Guard (USCG). If you plan on using your boat only for recreational purposes, a license is not required. Most boaters seeking a captain's license receive the Operator of Uninspected Passenger Vessels (OUPV) License, which allows captains to charter a boat up to 100 tons and carry up to six passengers. This license can be upgraded to include larger boats with more passengers with a few extra requirements.