Is there a reason you're looking for a portable? I had a nice Humminbird setup on my john boat that worked great. It wasn't hard to install and once set up I never had to worry about it. I removed the display unit when done fishing and just plugged it when I was ready to go out again.
I bought a Humminbird PiranhaMax Portable fish finder for my kayak. It has a suction cup transducer. It can supposedly shoot through the bottom of the kayak, but I've only tried it attached to the outside so far and it seems to work pretty nicely. A lot of the reviews on it are that it doesn't find fish. I can vouch that it didn't pick up very many fish, but it showed me the contours of the bottom and any structure, which is what I bought it for. It comes with a rechargeable battery and a carrying case.
I used mine yesterday. It worked nicely except in the cove under 3 ft where the weeds were almost the surface. It is powered by 8AA batteries.battery holder and 9 volt connector were less than $5 at radio shack.