My personal fave from a software/ease of use perspective in the Garmin range, and I would say forget colour displays in an open boat.
I currently run a Garmin 235 Sounder (GPS map/fishfinder/sounder all in one with mono display) which had a large portrait screen about 6" x 4.5". It's an excellent tool.
I think they're now called a 238 (with sonar) and 232 (without sonar)
"I Agree with everything you say really!" - John Cooke to Jon Fuller - 26-01-2013