...The 400c has charts already built in for the Caribbean, but they are very inaccurate (I'll demonstrate this below). I've debated buying a microSD upgrade from Garmin, but I have no idea whether it's any more accurate than what I already have. The inaccuracy is far worse than could be accounted for by the 3-year age of their chart. Why would Garmin intentionally put a grossly inaccurate survey into this device if they had better charts to provide on the chip? It just does not make sense, and as a result I'm not gonna buy a chip to find out. (I've heard that Garmin are absolute bast@%&$ about refunds.)
Here's how I did an informal test of the chart accuracy in the BVI:
Last December we went down for a land-based vacation on St. John USVI, and chartered boats a couple of days. One day we had a 23' powercat to go over to the Baths in BVI. Due to large swell and heavy chop, the captain hugged the coastline for much of the trip. We kept the Oregon 400c in my wife's purse to capture the track the whole day. His shoreline-hugging course gave me an outstanding opportunity to test the accuracy of the GPS and its embedded charts. I found many instances where the recorded track showed us significantly inland, and others where stopping points at shore were significantly off the shore.
For example, upon entering English waters, we had to check in at Jost Van Dyke's Great Harbour. I have exported this track to different charts to get an idea of the accuracy of the underlying charts.
First, the Oregon's embedded chart was off by almost a half nautical mile! The government dock where we stopped is about where the "D" in "Dyke" is located on the Oregon's chart, but you can see that our stopping point below is about 0.5 nm away from that point:
I exported this track and overlaid it on NOAA chart #25641, a 1:100,000 representation of the area (soundings in fathoms). Obviously the scale is not conducive to great accuracy, and I have overzoomed the chart for my screenshot. But you can see here that in this chart, the chart's government dock is located about 0.2 nm to the northwest of where we actually stopped:
...Finally, I found much more modern (but still somewhat dated) NGA charts (soundings in meters), which NOAA allows you to browse free of charge but does not make available for download. Here is a screen shot or our track overlaid on NGA #25609
, which appears to be much more accurate based on our stopping point and shore-hugging track:
This chart's survey was done in 1986, so it's much more recent, though still old enough to be cause of some caution...