These work pretty well, though they're noisy.
Took a dozen sticks, taped them together, hooked up a blasting cap, and threw it in the water right under his boat.
One of the witnesses was a Navy vet who'd taken part in a nuclear depth-charge shot in 1962. He said it looked just like a small-scale version of this:
. . . except for the fact that what was left of the boat was on top of the explosion.