The May 2010 issue of Light Aviation, the journal of the Light Aircraft Association, featured a chap with a Kitfox (2 seater homebuilt aircraft) converted for float operation. If memory serves he operates from a harbour in the south west UK. The Kitfox is a folding wing type, which can be trailered home for cheap hangarage, and the chap launches it from a harbour slipway using a quadbike to drag the trailer. He has been doing this for some time so presumably it is allowed.
Rans6