The OPP say the incident took place late Friday afternoon just before 6:00 at a marina.
Police say a 5 meter long Hurricane was being refuelled when the gas line broke, causing the hull to be filled with gasoline.
The boat would normally hold 200 liters of gas, but took on 360 liters before fuelling was stopped.
When the operators tried to start the boat, there was an explosion.
Three people were taken to Lake of the Woods District hospital and later released, while two others are still being treated for 2nd degree burns.