You need a US Coast Guide license (OUPV or Master) if you are:
Carrying passenger for hire (boat size and passenger count does not matter) if you are accepting money, tips, barter etc., from passenger you must have a Captains license, this applies on the Great lakes, inland waters and federal waters.
Operating commercial vessel if you are towing, ferrying, sightseeing, charter fishing require a licensed captain even without paying passengers. for vessels inspected and certificated to carry more than 6 paying passengers , you need a Masters license of the appropriate tonnage

You do not need a captains license, if you are renting or borrowing a boat for personal use only but you still must comply with state boater-education requirements. check state specific requirements