It wasn't just Marni—even Xi Wei couldn't sit by and watch in his divine kingdom.
He was unable to do much lately due to his lacking divine energy, and simply watched as the Players ran off to farm swamp fishmen. There was nothing he needed them to do anyway once he thought about it: things were normal at Fishmen Island, and the Ocean Goddess certainly wouldn't suddenly pop up at Lancaster several hundred miles away to stir trouble, right?
Therefore, the rampage of the swamp fishmen had to be a natural phenomenon. Creatures of this world would often suddenly breed in wild numbers due to the climate or some sudden factor, but though things could prove problematic afterwards it at least wouldn't hurt the balance of ecosystems—one way or another, nature's cycles and conditioning could be observed.
Hence, Xi Wei decided to break out his popcorn and enjoy the show when he confirmed that it had nothing to do with the Ocean Goddess' trial.