I went back and check an old post in hopes it had details of the transducer cable splice, but alas it is a bit skimpy on details.

In Feb. 2007, I did the cable splice taking out 10-12 feet of transducer cable.  Once the cable was cut, I removed the rubberized cover from each end for a combined length of about 4".  Then I carefully unwrapped the foil shielding and laid it back down the length of the cable on each side.  (Before doing so, I slid a 4-5 inch length of shrink tubing back down the cable and out of the way.  (Shrinking this over the splice would be my last step in the process.)  

I then measured each of four color coded cables and then cut each one on about a 3/4" stagger with about 1/4" of bare wire exposed for teh soldier joint.  This accomplished a couple of things ... it made the soldier connections in different places and minimized any potential line-to-line signal bleed.  So ultimately the splice points looked something like:

-----/-------------------------
----------/--------------------
---------------/---------------
--------------------/----------

For each splice in turn, I first slide a smaller diameter shrink tube over the longer end of the line to be spliced.  Then I soldiered the line back together.  Finally, I slide short section of of shrink tubing back over the splice and hit it with a heat gun.  (I think a hair dryer would do, but I already had the heat gun from time spent crafting remote control air planes).    I simply repeated the process for the remaining lines.  When all lines were spliced and protected by shrink tubing, I then re-rapped the foil shield.  The final step, was to re-position the larger section of shrink tubing and hit it with the heat gun.

I'd recommend that you make the splice relatively close (a foot or so) to the plug end.  In this position, there should be a little chance it would get submerged in the water (just in case the splice is not water proof).

dd