Why do high-speed water impacts / ditching usually result in structural loss of the airframe?
Plywood sheets can penetrate tree trunks unscathed


The plywood isn't unscathed by the process




  Why wouldn't an airplane [be unscathed by] impacting ocean water at a steep angle at 300-500 mph?


The aircraft will not travel at that same speed underwater. That means that a considerable force is applied to the aircraft to decelerate it in a short distance. That force greatly exceeds what the structure was designed for.