Re: BVI chart/nav app
TakeFive, I bought an Oregon 450 2 yrs ago in preparation for a trip out of the USVI to the SVI. The 450 didn't have built-in detailed charts so I bought the US and Southeast Caribbean BlueChart g2 chips. The SE Caribbean chip is # HXUS030R.
I found the accuracy to be as good as the paper charts for the area. We threaded some reefs at Culebrita and elsewhere using paper charts and eyeball navigation, including reading the depths, until we established trust in the Garmin system. We ranged from St Thomas to Culebrita, Culebra, and Vieques and back by way of St. John with no issues. Actually, it was rather impressive.
BTW, I took a look at Great Harbour using my SE Caribbean chip and it provides a little more detail than your map views. I don't know if this microSD chip works in your 400c, but I would think so.
Your 400c has the same dimensions as my 450, which fit nicely in a drink holder on the boat we chartered.