Casco Bay Boaters Blog has an interesting post - GPS Not the Answer
Most mates know not to rely upon GPS alone. On the other hand I sailed with a captain who forbid the use of the GPS while in confined waters, I don't think that is right either.
My view is, there is no navigation situation where I would completely abandon the use of the GPS, with the caveat that I would never rely upon it entirely.
In the open sea we use GPS but verify the position with a comparison to the DR. At the pier on the other hand, if we want to know if we are still alongside, we go out on the wing and have a look. In between, on the transit from the open sea to the pier, the role played by the GPS shifts from being the primary method (with verification by other means) to becoming a means to verify the position obtained using other methods.
As we approach the coast, at some point we shift from the GPS as a primary source to the use of radar and other means but still continue to use the GPS to insure we have not made a gross error. Even in a narrow channel I may not completely stop using the GPS, for example when I see that I am being set by the wind or current, a glance at the GPS course over the ground (COG) will quickly verify what I am seeing visually
Even relying upon the GPS as a backup however can hinder the ability to see clues in the environment that can aid in spatial orientation. As I gain experience in an area it becomes easier and easier to orient myself with visual landmarks. With each transit of an area I try to decrease the amount of assistance required from the GPS and increase my familiarity with local landmarks