I will concur. In my experience with this sort of thing the most difficult part is dealing with coordinate systems and datum. There are a lot of really obscure local datums that have been used threw the years. Also, even for pretty recent maps the accuracy of the map depends on the survey.
From From a NOAA page about chart accuracy and precision.
"Positioning of Survey Data
The methods used to acquire the underlying survey data will also affect accuracy. NOAA has specified stringent accuracy standards for collection of data by its survey vessels. Currently, surveys are being conducted to DGPS accuracy. However, for surveys performed prior to the mid 1990’s, the accuracy requirement was only 1.5mm at the scale of the survey as few surveys of years past were able to match DGPS accuracies. For example, on a 1:20,000-scale harbor approach survey, an accuracy of 1.5mm equates to 30 meters. NOAA makes every effort to produce the most accurate chart possible given the available data. The prudent mariner should pass shoals or isolated dangers with utmost caution, no matter what navigation method is used."
That is not to say you cannont get good results just that you have to be careful.