Post the make and model of the fish finder - can't see from the picture. Even better, post a link to the manual or datasheet for it that has info on what ports are available on the fish finder. Or a datasheet for the transducer if you want to connect it directly to the Arduino.
How about, I'll just use the 200khz transducer in it, then I will build my own circuit that will drive the transducer and be readily available for arduino interfacing? Can you help me with that?