Many thanks to everyone who replied.
The cable was cut, shortened to a metre in length and spliced successfully.
Inside the cable we found a braided uninsulated wire and two pairs of insulated wires; red and black (probably power), and green and white (probably data). Each pair was wrapped in foil shielding. After cutting the cable each wire was soldered and the join covered in electrical tape . We wanted to use heat shrink but it proved too fiddly. The shielding was too delicate to solder and there wasn't enough to cover the soldered and taped joins so we wrapped each pair of wires in kitchen foil. The whole thing was then covered in self-amalgamating tape.
The echosounder works fine and although we haven't rigorously measured its accuracy it seems accurate to at least within 10%.