actually for extreme simplicty try one of the Hummingbird Matrix's. i put one in my small boat cuz it was cheap and i didnt want a full featured unit in there. i think i actually used it more than the full maping unit i had in my Scarab. it also has depth and water temp on it. the GPS side of it is pretty simple with just the basics, walking map, bread crumbs, speed, compass, etc. . . . real easy to read as well.
i shoped around and got one for around $200.oo