https://teamkitfox.com/Forums/thread...=kotm+nov+2009

The above link is to a nice write up of one of the members Kitfox IV on amphibious floats. In the article he mentions that the empty weight of this airplane is 779 and it is a 1200 gross weight Kitfox IV. That leaves 421 pounds useful load. He mentions that the airplane has some extra weight in terms of paint, so a few pounds might be saved by using minimal coats of paint, or something like oratex for covering.

I have a Kitfox IV project that I would like to someday put on amphibious floats. Using the above as a guide and with myself and wife total weights at 330 pounds leaves 91 pounds for fuel and baggage. I wouldn't anticipate needing baggage if she were flying with me so I could get 15 gallons of fuel in which would be plenty.

While this amount of useful load isn't winning any awards it could very well be enough depending on you and your mission, and this is for the Kitfox IV. For the newer models, keeping weight as low as possible during construction would yield even more useful load.