>(passengers and cargo are loosely coupled to the hull and are sprung)

This is not true. The mass of the contents of the kayak are coupled enough that their presence will significantly determine how much damage will happen to the hull on impact.

I have hit things hard enough to wish I was loosely coupled, and I much to my regret I was not. It hurts. Me, and the boat. While I may have moved slightly on impact, I was by no means "loose" or "sprung".

The reduced weight of a lightweight boat will do next to nothing to decrease the inertia when impacting an immovable object.

Try kicking your boat as hard as you can. Do it empty and do it with someone sitting in it. Empty, it will go spinning and rolling away whether it is 30 or 40 pounds. With another 150 to 200 pounds in it, you will A: damage the boat or B: break your foot. The weight of the passenger and cargo are tightly coupled.