There are a lot of reasons to appreciate the Game Boy, and one of those reasons is "It has some of the most wackadoo accessories ever." You probably already know about the Game Boy Camera (a digital camera before its time) and the Game Boy Printer, but did you know Japan got a fish sonar attachment for the grey brick in 1998?
Gaming Historian recently looked at the Game Boy Pocket Sonar. It's essentially a modified game cartridge connected to a long wire that feeds through a yellow bobber. If there are fishies below, you see them on your Game Boy screen. You can also get information on the depth of the lake, read a fish encyclopedia (all the information is in Japanese, sorry), and play a simple mini game wherein a little man must manage several lines at once.
Norman Caruso, the titular Gaming Historian, even tries the sonar for himself on a lake and catches what appears to be a bass. If it's your dream to catch fish with the aid of your Game Boy, there you go: The Pocket Sonar checks out.
There are limitations. Again, the sonar was never released outside Japan, so it might not do you much good if you don't have a rudimentary understanding of the language. The attachment also isn't compatible with the Game Boy Advance or Game Boy Color, though it is compatible with the Super Game Boy attachment for the Super Famicom. At least, partially: you can access the fish encyclopedia and the fishing mini game, but it's probably going to be hard to measure a lake's depth unless you throw your Super Famicom onto a lake. (Don't do that.)
Stand-alone fish sonars can run anywhere from a hundred dollars to several thousand. The Game Boy Pocket Sonar isn't the fanciest alternative, but as a unique bit of budget hardware, it's pretty cool.