Re: BVI chart/nav app
2 years ago I bought a waterproof Garmin Oregon handheld chartplotter along with a chart chip for the eastern Caribbean. This was in preparation for a bareboat charter that included the Spanish Virgins. I also bought waterproof charts and a Seaworthy guide to all of the Virgin Islands so I could lay my hands on the navigation data and just in case the electronics crapped out.
An app might be nice, but if you want something you can rely on under wet conditions a small handheld is the way to go when you are the skipper on someone else's boat. You can set up waypoints and routes ahead of time and familiarize yourself with electronics that you'll be able to use when/if panic sets in. If your charter boat has a chartplotter, at least you'll have a backup that may be more user friendly (if you've done the drill). One advantage of the Oregon--and similar sized Garmin units--is that they fit into a drink holder by the helm.
The higher end Garmin handhelds are not cheap, but they have more pixels (higher resolution) than the cheaper Garmin handheld chart plotters and they obviously are a lot more rugged than a cell phone or tablet. Think of it as insurance--as part of a layered approach to assuring your safety. The only drawback is that they don't float.
I rationalized the purchase as a backup for my own boat's installed chartplotter/GPS which would fail in the next lightning strike. (After losing all my electronics twice to lightning, this is more than a hypothetical situation for me.) The advantage of handheld electronics is that you can protect them by putting them in your oven or wrapping them in aluminum foil when lightning threatens.
That said, I am trying to figure out what charting and weather apps to acquire for my iPhone (and maybe an iPad in the future) and hope to get some suggestions from this blog.