A passenger for hire is any passenger that paid money or contributed anything of value (barter) as a condition for being a passenger. If you carry even one passenger for hire you need a professional captain’s license, such as Master or OUPV. Both licenses will allow you to carry passengers for hire and both will allow you to carry passengers on ocean waters. The difference is in the type of boat that you are operating and the number of passengers you are carrying.
The OUPV or “Operator of Uninspected Passenger Vessel” license allows you to operate one type of vessel; a small uninspected passenger vessel carrying six of fewer passengers for hire. A “small” vessel is defined by regulations as a vessel less than 100 gross registered tons (GRT), which is a measurement of volume not weight. In general this would be a vessel less than about 70 feet long.
The Master license allows you to operate a small uninspected passenger vessel, but it also allows you to operate inspected passenger vessels carrying more than 6 passengers, as well as crew boats, supply boats, oil spill response vessels, cargo vessels, ferry boats, research vessels, or any other type of commercial vessel other than a towing vessel (that is another post).
If your objective is to carry 6 passengers for hire as a fishing guide than the OUPV is all the license you will ever need; but if you have your sights on carrying more than 6 passenger or operating other types of vessels you should consider either getting a Master license or upgrading the OUPV to a Master.