This is why I love gameSniped, as I have never heard of this until now.
The Game Boy Pocket Sonar is a peripheral for the Nintendo Game Boy made by Bandai that actually used sonar to locate fish. The device retailed for 14,800 yen ($186 US) and had a fishing mini-game built in.
It consisted of a gameboy cartridge to which was attached a reasonable length lead, at the end of which was a cylindrical object, that just so happened to be the sonar itself. The user would dangle the sonar in the water where they expected to find fish. The sonar would emit a sonar frequency, and the gameboy screen would show what was found.
The product was developed in cooperation with Honda Electronics Co. and was marketed as costing about a third as much as a comparable sonar device. I bet it only worked a third as well too.