When I replaced my Raymarine speed transducer, I soldered and sealed with heat shrink tubing the end of the new cable to the end of the existing cable. I did this rather than snake the new cable back to the pedestal. 

I just did the same with a new depth transducer. The electrical conduits on our boat are jamb packed with existing cables and wires very tightly bound together with cable ties. Pulling a new cable thru the boat length with the old one was impossible and I didn't want to remove all the cabinetry and drill new holes to feed it. The Airimar transducer came with a cautionary note to not shorten the cable,  and If more cable length was to be added, use a special water tight junction box, which I could not find on the internet. I suspect shortening the cable might change the impedance designed for the system, but I assume  lengthening the cable by soldering or junction box would not adversely affect the performance. 

Has anyone else simply soldered the cables and did it effect depth sounder accuracy?