He's not using some sort of electrical ray to make corpses swirl around for his amusement, although that's what it looks like.Oh. let's dance, my pretties!
They're corpses from hundreds of years ago, lost when the galleon sank with the usual amount of gold on board. He's in a new sub, the Dolphin; we learned at the end of the previous adventure that the Pearl had suffered too much damange to take to sea again.
He's starting to go through subs like airships.