I've read and re-read the rules on fires and that's what it appears to say. Any fire will cost you a minimum of two full damage boxes. As you said, a fire onboard is always a priority and I'd think it would be especially so on a ship made entirely of wood. Maybe I'll have to house rule it and say if the repair is the only action performed the following turn that it can be accomplished in one turn, limiting the damage to only one damage square. Of course the penalty would be that nothing else could be accomplished that turn.