A Close Look at the Options Available to get a Sailing Captain License for Leisure or Commercial Purpose
If you ever plan to venture off shore by more than 20 miles (32 Km approx.) on keelboats 26 ft (8m) and above, the charter companies may ask for a license along with your logbook to look at your experience as a sailor. There are several options for you to get a sailing captain’s license. But remember, the sailing experience is the main criteria and a license is added so that they can test your knowledge on local laws and everything about the vessels, weather, electronics etc.
If you are in the United States, you may look into getting a USCG Operator of Uninspected Passenger Vessels (OUPV) if you want to take up to 6 passengers on your vessel or a Master (Up To 100 Ton) License if you want to take 7 passengers or more. You may also take a Sailing Endorsement for sailing vessels - It requires 360 days of sea time aboard a sailing vessel. A sailing endorsement is required if you want to operate an inspected sailing vessel. You may look into online programs & online exams like Mariners Learning System or physical classes and, exams at a USCG regional center (REC).
You may also check American Sailing Association (ASA) and United States Sailing Association (US-Sailing) programs for sailboat and powerboat training options with certifications accepted in the U.S. There are only eight states that do not require you to have a license to sail a boat within their waterways.
Private charter companies accept license issued by private sailing schools like NauticEd which are accepted internationally. They have a Captain Rank program which you can complete online. Along with 50 hours experience logged on their electronic sailing log, they issue a Captain Rank license. If you are planning to take a bareboat out in the ocean, that option is also available (Bareboat Master Rank).
UK & Europe
If you are in the UK or Europe, an RYA license would be a good idea. Most of the theory classes can be done online through schools like Splash Sea School like Day Skipper or Yachtmaster and you may get trained for the practical and pass all exams at the training center. You can also join mile-building trips to complete the requirements of a license.
And, if you own a boat in the U.S., there are boating licenses issued by states. Each state has authorized NASBLA as the sole organization to issue a boating license. Thus, if you hold a NASBLA state boating license then you, by default, hold a USA boating license. Similarly it is PCOC in Canada, and other boat licenses issued by respective countries.
So, it all depends on what you want to do with your license really!