This is why boats are a bad idea all around, even if they do come with convenient blunt weaponry already primed for murder. Going overboard is too easy an escape for Zagreus' tastes, and he makes the ill-considered decision to try and haul Johnny out of the water, leaning precariously to snatch at his collar. It's not like Johnny could just slip under the surface and swim away, even if he were inclined; the horrible glowing whatevers have seen to that, activated with Johnny's helpless flails, making him an easy target. Zagreus is determined that he should pay for marking him, even so trivially. This is such a terrible idea.