"Substantial damage" from an insurance standpoint, perhaps.
You saw the pics of the plane floating in the water, cabin intact, wings still in place, rudder, stabilizers, tail section intact, engine still in the mounts, landing gear extended and in place.
IOW, pretty much in one piece, despite the fact that it would be unlikely the plane would be recovered in any condition to be made flyable again.
For insurance purposes, the plane was a write-off and had suffered "severe damage" from the standpoint it would be uneconomical to fix and return to service.
Physically, however, all the major parts were pretty much where they were supposed to be.
When they hauled it up from the bottom, it looked like a pile of junk. Hard to believe the surf could do that much damage to an intact aircraft.