I've only ever used a Garmin GPS Map 76, which I like a lot. It won't
accept sounder data though. The next Garmins up , both the GPSMAP 276C
and the GPSMAP 376C support external NMEA sources for water depth,
water temperature, and water speed through two serial interfaces..
They have built in Lithium battery packs so they don't need external
power and are optimised for either Auto or Marine uses. See:
http://www.garmin.com/products/gpsmap276c/