Does anyone use a portable fish finder? I usually fish out of a jon boat and sometimes I go to places where I have to rent a boat. Can I get some input on what you guys use/recommend?

I used to have a Fishin' Buddy II. It was good for what it was I really liked it. My wife got me the Fishin' Buddy 4200 for xmas 2006. I have to say it is great. It had all the functions of a non portable unit. I would recommend it to any one.

http://www.cannondownriggers.com/site/html/htmlsite/bl/4200.htm

There are also a few good Eagle units out there. I have a Eagle 242 that is portable and works great for the conditions you described and I even use while ice fishing.

Harshman

Hummingbird Pirhana Max10 or Max30.They're good basic units that you can use to find structure,deep cover,baitfish,fish,water temp,and bottom composition.They're cost efficient and are very simple to operate.

Humminbird PirahnaMax 20 is a good one.  I bought one last March and used it all year without a hiccup.  Runs on 8 AA batteries, so invest in some good rechargable batteries.

Cabelas offers several models that they do in house.  Comes in a nice nylon carry bag with a suction cup on the transducer.  I think most of them run off of a 6V lantern battery.  

Consider the battery before buying anything.  With the PMax20, I would get 8-9 hours out of a charge in warm weather, considerably less in cold weather.  You want something that takes a battery you can recharge.