Chris,
I have been using the M68C from Lowrance. GPS mapping is great for La Jolla (I have used throughout CA fresh and salt) and U.S., but sucks down in Baja - shows me on the beach when I am 1/2 mile away - evidently a Mexico issue, but Lowrance won't get back to me with a fix. Has been great for
in bad visibility, puts me right on the hoops. The color graph is fairly good. I like the slim design, but reduces detail and visibility simply because of screen surface area - especially when you use split screen for map and sonar at the same time. Main flaw is the transducer cable - the rubber around the plug rots from electrolysis, even though I use the electronic grease. If you go with this unit, I would suggest trying to find away to seal that area of the unit from the salt. Otherwise I am very happy with it, only goes to about 400 ft through the hull in salt though.
Take my opinionl with a grain of salt though. I am not an expert - I just play one on the message boards.
Richard