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I was a bit surprised to hear "victims broke their necks instantly," so I was hoping for someone else to do some math to back it up. Sigh...

Assuming ≈1000 ft/lbs to break a neck, and a 220 lb 6'10" person (so we can exclude the head to get ≈ 200 lb 6'), if it were possible for them to keep their body completely rigid, simply laying them horizontal, only supported by their head in a fixed orientation, would produce ≈ 600 ft/lbs (CoM 3' from the moment) on the neck. Which is actually a surprising testament to the strength of the human neck.

The actual math for determining the torque on the neck from a worst-case scenario of a person jumping off a moving boat and hitting the water head-first with a stiff vertical posture is a bit above my pay grade. You'd have to calculate the resistance of the water over time as the head makes contact and starts deflecting, vs how much of that force gets transferred to the body and starts to rotate it towards the water.

But just conceptually, doesn't make a ton of sense to me, as someone with a fair amount of experience with hitting the water at high speed; I used to barefoot water ski. Sucks hitting the water that fast, but the worst injury I've sustained is a ruptured ear drum.