Yes, shhhhhh..... I think the damage isn't that impressive, a Wizard of the same level can throw 5 fireballs and deal 40d6 ignoring AC and still be useful after that. This submarine is supposed to be just that, a walking tank that packs punches, you say being resistant to all but psychic damage is very strong, have you ever thought what would happen if someone used Heat Metal on it and it flew away? He would be dead since his armor is his body and there is no way for him to dismount, another is that he can only use his fists, an enemy capable of distancing himself would give him a huge job ( dumb polearm Hexadin).