Sooo... exactly how much give should I expect in the exterior hull? I can push in a bit. Feels like the fiberglass is solid but there's either no backing or possibly rotten backing. Is it possible the hull is solid fiberglass with no wood interior or is this thing just trash?
that really depends on how thick the hull is, and the type of resin used. I've had fiberglass flat bottom john boat style hulls you could push in with little effort, the sides were a little over 1/8" thick and epoxy resin.
If it's a single layer under 1/4" I would expect deflection.