Just my two cents here, the Mer-leader told you to throw the ball into the ocean (not fire it from a cannon). I'm pretty sure this was just a case of over thinking the puzzle, but it wasn't that difficult for me to figure out there was a hotspot called "ocean" by the no fishing sign. Yes they shouldn't have the invisible LeChuck at the cannon or the rubber tree back in place, but you'd never even see those if you just threw it in the ocean like you were told. I'm going to have to say you're just being too picky here.
Here I am, brain the size of a planet, and I'm sitting here responding to posts.