Somebody used to make a "portable" fish finder which just clamped on the side of the boat. Sucked for finding fish, but I found it worked pretty well for telling the depth. By using it to find the points and dropoffs I was able to catch fish with it. Best thing is it required no install on my 10' little aluminum boat and ran off of C batteries. Heck, it's probably in the garage still..
Ah, found it online. Bass Pro has it, the HummingBird 110. Priced at $130. New ones run on 6AA batteries.
http://www.basspro.com/Humminbird-110-Fishin-Buddy-Ultimate-Portable-Fishfinder/product/97460/Would NEVER recommend it for any serious fishing, but if you don't want to lug around a car sized battery to power your fishfinder, then it's a decent option.
