depth sounder transducer cable repair

Where did you get the idea that you can't splice transducer cable? As long as you make a good connection - like soldering the individual wires - there is no issue other than the lack of waterproofness of that portion of the cable. Also, if you leave a large area unshielded, and the unshielded portion is close to a source of noise - the ignition system, for example - you will pick up more noise. The signals you are dealing with are audio frequencies (all right, VLF RF) - usually between 50 and 200 kHz. The length of the cable makes little difference, as the speed of sound in water (what the sounder is actually measuring) is around 4,814 feet per second at 60 degrees F. The speed of the signals in the cable is in the neighborhood of

687 million feet per second (speed of light in vac. * a guess of .7 VP for the cable). A bunch of feet longer or shorter will not affect the accuracy, as the timing doesn't change significantly. If you were to add enough cable, it would reduce the sensitivity due to the attenuation of the transmit pulse, and the received echo to and from the transducer.