[BigLew]

With regards to chart plotters, first let me say my experience is in coastal waters. Having said that, aside from chart availability, consider very closely the way image data is stored.

There are generally two methods of doing so. The first, and most common, is the raster method. This simply means that in essence, NOAA charts are photographed at various levels of zoom and those images are stored on that chip and when you get to the most zoomed/enlarged presentation of the image- that's it. If you still can't visually differentiate on the screen your position from that which you want to avoid, you are is some trouble.

The second is a vector method, used by Garmin only to my knowledge, which essentially digitizes the NOAA chart image. That means that each point on the chart is assigned it's digital location relative to all the other points. What this translates to is that in situations where high levels of zoom are desired, the vector information allows the greater magnification of the associated data points.

A situation where this might be desirable is in a very narrow channel with hazardous obstructions very close by. If your zoom capability is not great enough then you may not be able to SEE, on the plotter, the very close distances desired in order to avoid those obstacles.

With regards to open water charts and your availability I believe that C-Mapp, Garmin and others have availability if you contact the company. I am not sure of that however.

I fully expect that some on the forum here are going to disagree with this in general. To them I say, I've seen it and I know it is in fact true. Recent boating has been on the Merrimack River in Newburyport Massachusetts. At the very mouth of the river and channel is extremely narrow and is bounded by a rock seawall on the north. I have needed to discriminate distances of 10 feet and less, in fog and have been able to do so with the Garmin unit. I have not been able to do it with a Raymarine radar/plotter unit. However, in those cases I have been able to use the radar image overlayed on the chart and have been aware of other boats by keeping close watch on the radar as we approached the mouth of the river.

Keep in mind that this particular discussion has focused on close quarter visualization ability and not open water chart availability.

Good luck.