Depending on the size of the passengers and their fishing guides and pilot the plane could easily have been over gross. Not only people are on board but fishing gear( which can be heavier than one might think), the guide's back packs which contain lunches, beverages, fishing gear, and emergency gear are all on board as well. And beavers burn around 25 gallons per hour. So fuel weight is also a factor. Add all that up and it can be easy to be over weight. Try to get airborne in a smaller lake with out a good head wind and the Beaver just might not have enough for a safe take off. Passengers can and should ask questions about weight and weather and if not happy with the answers should decline the flight. Nobody has ever been hurt in a plane that  crashed which they were not in.