Nothing dramatic. The mag compass might be "off" just enough to notice on a calm day when it is possible to steer a straight course. Usually, the boat is moving around enough that one doesn't notice, especially there in the mouth of the Kennebec, where I have had to correct by 25 degrees or more to compensate for the current. In other words, I think the "local magnetic disturbance" is insignificant relative to the other forces that affect navigation in that particular passage.