Thinking about it, another great thing to have done would be pour in a whole load of bleach into that tub full of Marisas and watch them scream, knocking each other in and melting away...
That'd defeat the purpose. This is a reference to the fishing game at japanese festivals, where the prize is usually just a small thing of some sort (usually a water balloon). Killing all the yukkuris with bleach would hardly be profitable, unless you charge people to watch it.
It wouldn't defeat my own purposes, still (and profit isn't really a concern with abuse). Of course, the thought that this was a reference/profitable game occurred afterwards... although I imagine there would be a good amount of people who would pay good money to watch such a thing, so there's money to be made there.
You don't really need bleach for make it a 100% torture game, just be creative and take advantage of every little aspect of the situation, water is fine, all you trully need to do is pretty much the same of the beginning of the comic, drop 1 in the water and make them aware of the danger, everything else is up to your imagination.