I saw this portable fish finder at Meijer yesterday for only $80. You don't have to drill any holes through the hull of your boat. The transducer is attached to a suction cup that you just stick to the side of the boat. (Pictured below.) You don't have to worry about wiring it through your boat's electric panel. It runs on just two AA batteries.

Humminbird Piranha Portable 1 Fish Finder