from the sounds of it (not wanting to add a battery) you are better off to use a hand held with a ram mount or something along those lines. I'd be surprised if there is a unit that is not hand held and runs on AA batteries.
I was in the same boat a few months ago. I wanted a gps that would run off batteries (not necessarily AAs, just battery powered) and came across a Garmin 640, reconditioned, for a very fair price. Put a ram mount ball on my center console and one on my IPB 14, and move the unit between the 2 boats with no problems. Battery life is good, and since I only run it about 5 hours at a time (on and off), it works well for me. Screen can be a little hard to see in direct sunlight, but like anything else, it's all about the angle. On my little skiff, I have it tucked under some cup holders on a grab bar, and it is shaded at nearly every angle.
I have a 640 with ram mount to sell. New only used a few times. Good unit and comes with box, manuals etc. I also have a back up lithium battery. I'm all in on it with ram mount and 2nd battery for close to 700. Selling for 500 and will ship.