Personally, I have a Garmin 4210 GPSMAP at the cabin helm networked to a Garmin 3205 GPSMAP at the outside helm. These are networked with to a Garmin GDS 22 dual signal digital fishfinder and a Garmin GDL 30A Siris XM weather receiver. I'm very pleased with this plug and play Garmin network. However, I believe that there are several fishfinder systems that have better functionality. They can be interfaced with the Garmin network through an NMEA 2000 connection. I advise avoiding the touchscreen GPSMAP consoles - they just don't seem to be as rugged and as easy to use as they should be, but maybe that's because I suffer from fat fingers and prefer real buttons. The only tricky part on the installation is the DSC interface between the 4210 GPSMAP and the West Marine DSC enabled VHF radio. Essentially, that is an NMEA 0183 network connection. I used a piece of CAT 5 cable wire to make that connection. I think Garmin now supplies that wiring. The Garmin software is easy to use, and generally very intuitive. Their support is excellent. So, I'm very please with the investment. I installed this system in the spring of 2006, and it is still going strong.
Hope this helps.