I got the impression you were looking for a flying boat. The Sig Sealane is the only Flying boat kit I know right now. You seem to think it is awful. I have seen a couple of them fly, and feel that it just needs spray rails on the hull for better water manners. It seems to fly pretty well, if you like a plane that flies on the wing. If you want to punch a hole in the sky, the Northstar is kitted by BalsaUSA. BalsaUSA also used to kit a Laddi Mikulasko design called the Laker, it may still be available.

Ace used to kit a Fred Reese design called the SeaDancer. Now they are Thunder tiger and they offer the Seamaster-but no longer in kit form as well as I can determine.

If you can get plans for one of these you can contact one of the kit cutter services......you might just call those guys anyway. Once they cut a kit they have the dxf saved in their computer.

If you're willing to try a floatplane, One Kit that flies well on floats is the good ol' Astro Hog.