Hi Take 5
The US Coastal does not cover the BVI's, you will need to buy the North America coverage. Your question about the Coastal chart accuracy is therefore moot.
As for overall accuracy, here goes (I know you know all this, but for the benefit of the peanut gallery);
1. With all electronic charts you have three potential errors, chart errors, GPS errors and the relationship between both. Without all three being 100% you are getting incorrect information. You have to assume that for pilotage, plotter info is not accurate.
2. I have used Navionics, Garmin Bluechart and Jeppesen C-Map in the BVI's and Caribbean. Which is most accurate? It depends. They all take turns and all claim to be the best. I tend to like Garmin because I drive on the white, slow down on the blue and stay off the yellow and green.
3. My favorite electronic instrument is a depth sounder. My favorite non electronic tool is a long boat-hook marked in one foot increments which I use to sound around my grounded boat in order to find the direction to kedge/get towed off.
4. These electronic charts are now relatively cheap. Buy as many as you can afford and have fun with them. If you really want to understand chart accuracy in the Caribbean you should have listened/read the discussions between Don Street and Chris Doyle about the accuracy of their relative chartlets.
If you have lots of spare time check this out;
Street's Cruising Guide to the Eastern Caribbean: Puerto Rico, Spanish, U.S. & British Virgin Islands - Donald M. Street Jr.