The mystery of Ben Gardner...

We see Hooper and Brody approach Ben Gardner's boat, which is drifting in the darkness. A search light is shone over the boat briefly showing some damage to the boat in the form of splintered wood. Ok.
Then Hooper dives under the boat to examine the submerged portion of the hull. He finds a hole in the bottom of the boat and a shark's tooth imbedded in the wood.
Ben Gardner's head, sans one eye ( I can't remember for sure which eye), rolls out of the hole and sinks into the depths.
Here's the mystery: How did Ben Gardner die? I cannot think of an even remotely probable shark attack scenario where the victim is operating a boat and somehow his head is the only thing left of him and it is inside the boat.
Here's a few possible scenarios to demonstrate:
Gardner is lying down in his boat at the exact moment when the shark punches a hole through the bottom of the boat, bites off his head, then spits the head back into the boat, presumably all in one motion.
Scenario 2: Gardner is leaning out over the water, the shark lunges up, bites off his head, then immediately or at some point thereafter, regurgitates Gardner's head back into the boat.
Scenario 3: Gardner is knocked out of his boat and as he is pulling himself up the side of the boat to escape, when the shark bites him such that his entire body is taken except his head (which must have been above the level of the side of the boat because it then rolls into the boat).
It is just really hard to come up with a scenario where Gardner is decapitated or his entire body is taken by the shark and yet his head remains inside the vessel.