I'd need to put more testing into this to determine if it's unique to boats or if it happens with land creatures too but here's the setup. On a boat, three characters on the same tile. Regardless of which of the three initiate combat the mob always targets the same character. Not a huge deal with sea serps, but I imagine someone attempting to steer damage to a toon that has resist vs one that doesn't the way the mechanic currently works is broken.
So interestingly, when the mob is out of range of magic/arrows, but in range of firebreath, it will correctly target the toon that attacked it. But once it gets into range it immediately switches aggro to the "designated" toon. The designated toon appears to be the first one to have entered the shared tile.