In short the Master is a superior license to the OUPV (Operator of Uninspected Passenger or 6-pack), but for a fishing guide the OUPV is all the license you will ever need. The good news is that the OUPV may be upgraded to a Master license with the same route by completing a three-day course.
The OUPV license issues the authority to carry six or fewer passengers, not counting the crew and operator, on small uninspected passenger vessels (less than 65 feet in general) on all federally navigable inland waters, and, if you qualify for the Near Coastal route, on the near coastal ocean waters within 100 miles of shore.
The Master license issues the full authority of the OUPV, and the additional authority to operate inspected passenger vessels carrying more than six passengers; and inspected crew boats, supply boats, work boats, or cargo vessels within the limitations on the license. This license is also valid on all federally navigable inland waters, and, if you qualify for one of the superior routes, on near coastal waters within 200 miles of shore, or on all oceans worldwide.