When my old datamarine died, I replaces it with a PiranhaMAX 170 Hummingbird Fishfinder. The instrument is mounted on my pedestal guard and the transducer is mounted in the engine compartment with toilet ring wax. It works great and even allows for finding fish. They are priced from about $80 to $100.
S/V Zydeco II, 1983 Catalina 27