Eldher

The "black screen material" is carbon fiber fabric, 2oz per sq yard. When set with an epoxy or resin it becomes a very hard, strong, and light material. Strength to weight is better than fiberglass, but fiberglass would work well in this application too.

The strength is for water operations. Both takeoff and landing pose a challenge to the doors and hull. On a takeoff run at 20mph the doors will be getting hammered. Especially if she skips on the run (i.e. becomes airborne momentarily and touches the water again). Even during the run, water will stream all around the doors, pulling on them. Landings will pose issues too. The doors again will come in contact with the water at high speed, even if you make a nose high touchdown. And we all don't make perfect landings
Also from my friend Bill (who supplied the landing gear) has a lot of experience with waterborne models having designed at least a half dozen. His experience shows that over time the doors take a pounding and need to be set up to survive.

As for the hull needing strengthening, it's only 1/8" balsa to start. Full scale PBYs on hard landings could pop rivets and split seams. While the forces aren't exactly the same, we only have thin balsa here. I normally finish balsa surfaces with 0.75oz fiberglass before paint. While this makes a light and smooth finish for paint, it adds little material added strength. Instead, 2oz carbon fiber will make the bottom of the hull tougher then aluminum. 2oz fiberglass would work just as well. I use carbon fiber fabric to increase the load carrying capability of many key parts (like the rudder as I mentioned earlier). Here it is superior to fiberglass. I have it available, so will use it on the hull.

Kind of a long answer, but I hope this helps.