Fishfinders Have come a long way. There's new technology from Humminbird that deserves a serious look for it's wireless technology. A portable sensor is imbedded in a bobber that you cast out ; the sensor sends "pictures" of fish and bottom contours to a small display screen. The screen can be located in a mobile station or mounted on a fishing rod.  It's even available in a wristwatch configuration.  Smartcast is a new technology for Humminbird, but it's gaining clout ; the manufacturer now offers it as an option on it's Matrix line of sophisticated, high-resolution fishfinders  Check out the (Field Test) PDF format.
If you need to Download PDF Adobe Reader (Click Here) It's Free!  (Become a Field tester and recieve Free! Humminbird Products).  Read about it in the field test.