[GPSNavX]

AIS Transponder is a good option and MacENC (and iNavX) can use it to navigate with (instead of a stand-alone GPS receiver). There has been a lot of debate lately about whether the "big ships" even display the AIS B Transponder targets. MacENC will display both AIS A and B targets.

Both a transponder and/or receiver can be connected to multiplexer.

As far as BT over USB, I prefer USB for most reliable connection aboard. BT does work, but if the connection is lost then MacENC must sometimes be restarted to reconnect.