Navigators using iPads or iPhones will most likely be using the GPS on the device and a downloaded version of the charts. I use Navionics and have been very happy with it in the UK - will be interested to see how it fares in the BVI in a few weeks time (yeah!). Looking over some of the Navionics charts of the BVI some of the contours are a bit other wordly (speaking as someone married to a geologist) when you zoom-in, so don't plan on relying on them too much (Team Vestas anyone?)

If you go this route - you need the 3G version of the iPad not just the wifi one, as wifi-only does not have GPS.