Lowrance HDS fish finder plotter wont power up. Testing the power cable. The joy of boat sea fishing

Well my father couldnt get his Lowrance combo unit HDS to turn on. So he checked the battery and fuse and it was fine. I then went around and got my multimeter to test the power cable. The cable was reading faulty. There either had to be water ingress and corrosion in the plug, or damaged insulation. I cut the cable back several times, but couldnt clear the fault. I believe water has got in the plug, corroded the positive cable badly, and shorting out both positive and negative cables. A new cable is on order, all I will have to do is take the NMEA plug off, and solder it on the new cable for a gps input to the Icom VHF radio