Finn started nervously scratching his gun toting arm as he considered the possibilities. "Perhaps it was an accident? White Fang turn up, or someone on the ship snaps. That creates a wave of panic, brings Grimm on the ship, all hell breaks loose? First off aquatic Grimm, then, as they got closer to wreaking, land based Grimm hopped aboard maybe".
If Grimm weren't involved in any way, then it meant they were dealing with a particularly vicious individual or group, that wasn't afraid to make a mess. And, quite frankly, that terrified Finn more than anything.