For a split second Sam thought it was a leg cramp.
Then the beast got a better grip and pulled him under the surface of the water and he understood.
He was meat.
It had him ten feet under before he could get his knife out. Another ten before he managed to close with it and drive a killing strike into its single bulbous eye.
Then, lungs screaming for air, Sam released the corpse and struggled toward the surface, left leg hanging limp.
With luck, he could make it back to the boat before their combined blood drew other predators.