I have navigated around the USVI and SVI with an iPhone, but I also had my Garmin GPSMAP CSX760 and the chart plotter on the charter boat. I was using the iPhone and verifying accuracy with the other two units (and paper). It works pretty well.

EarthNC Marine charts is what I've been using.
I download all the charts of interest in advance over WiFi so I don't need a connection when away from land.