you might want to look into the sonarserver wifi unit. it works with the navionics boating app. it is a wifi repeater that send standard nmea 0183 signals for depth and gps to the ipad over wifi. that gives the navionics app gps info to display on the app chart and enough depth info to make your own sonarcharts.
you can use it with pretty much any head unit that has a nmea 0183 output which means nearly any plotter built in the last ten years or more. you connect it to the loose wires on the power cord the same way you connect the gps for a dsc radio.
http://sonarserver.com/
