Short hops or auxiliary fuel cell in cabin. I saw an episode of Ice Pilot NWT where they sold one of their water bomber to a company in Turkey and it documented the delivery flight. Fuel bladders with battery powered pumps were placed into the water tanks. What was more interesting was the use of handheld Garmin GPS for navigation over the North Atlantic. Everything was visual and daylight flying.
No need to account for weight of freight or passengers so the fuel load can be quite a bit higher.