I've never flown a seaplane but is on my list of fun airplanes to build. I've seen several kits similar to the Wave King. The Sea Bird 600 by Du-Bro. The Wavemaster by King's RC Distributors and the Harpoon by J. L. Modelcraft. These all had a plastic fuselage except for Harpoon with fiberglass fuse.
I found some pictures of a Wave King. It's not a copy of the Wavemaster but maybe a copy of the Sea Bird 600. The Wavemaster is more streamlined. The windshield has a more laid back angle and the engine pod is thinner and longer.
I especially like the Wavemaster and have been searching many years for a kit. I was lucky enough to find the plan along with a few plastic parts for the motor pod and tip floats. The plan isn't full size but could be enlarged for scratch building. I like your idea of making a fiberglass fuselage. I might do the same.
Attached some pictures I found of these kits. Please post a picture of your Wave King.