Found this reply elsewhere, granted the contact details are US numbers, but goes someway to answering your question.....
This is AirMar's answer to the question of cutting the transducer cable. They make most of the transducers that [are used in recreational SONAR units]:

Can I cut my transducer cable?

Yes, the transducer cable can be cut. However, if the transducer came with a connector do not cut it off. The molded on connector is waterproof. You need to cut and splice the cable away from the connector using Airmar's splash-proof Junction Box. The connections will not corrode and the strain relief grommets are water resistant and have excellent cable retention. Please note that cutting the cable or removing the connector, except when using Airmar's junction box, will void the sensors warranty. You can buy a junction box and splice kit for our distributor Gem Electronics.

Gem Electronics

Phone: 843-394-3565 Fax: 843-394-3736.



Note: Carefully follow the instructions provided with your junction box.

***If you have a TRIDUCER multi-sensor, the ceramic element inside builds up a charge. When the cable is cut, it can discharge possible disabling the speed sensor.

The cable you have should have the cable details on it so you can find the right stuff to splice in...

 

This Junction Box is about the same cost as the extension cable, think I would go with the extension cable as it's a lot less hassle and it will look neater

 

Steve, you are probably right, but if their only concern is keeping it watertight and vibration proof, then splicing in a new piece of cable with good waterproof joints should theoretically be good. If the unit isn't still under warranty then there is no issue there, the only prob would be if the unit developed a fault and had to be returned under warranty, regardless of cause, it would be your fault...
Doing a bit of homework should turn up the right parts to make a proper extension lead too, although crimp pliers may make this optionunviable inless you plan on selling a few...

 

But doesn't the extension cable just connect to the existing connector and it should have an IP waterproof rating, so the extension cable is a quick fit and should be waterproof

 

Yup, but, I agree with the op, £70 for 3ft of cable is way over the top, then you've got to lose the other 7ft somewhere...... Airmar have said joins are ok , so why not do it that way for a fraction of the cost?

 

I don't know if you can get the Airmar Junction Box in the UK, or how much it is, but if you buy it from the USA it's $65 plus postage, I would say the extension cable is better value and losing 7ft cable is easy if you coil it and tie wrap it ....... just saying.

However if you can join the cable without any of this expense then yes I agree with you and to be honest you can make it waterproof by using some shrink wrap with glue

 

Surely if the joins are soldered and covered with heat shrink it would be ok. Joint will be in the wheelhouse anyway so weather can get at it.

Peeves me off cos it's only a quicksilver 640 and the bloody transducer cable is so short, just another way of getting more ££££ out of you.

 

I bought a secondhand Garmin 350 colour the guy who removed it had cut the transducer wire to remove it... I just peeled enough cable back to see all wires fully. Slid shrink tube on both ends.
Re-soldered all wires in order and wrapped the shielded wires in foil as I found them.
Slid heat shrink back over all joints and warmed.
Works perfectly fine crisp and clear images.

 

Fellas , all it is is a waterproof electric box with grommets, like a switch backing box, you connect the wires to a chocolate block inside.
I bought a b175h chirp when it first came out, with a garmin plug, I had to buy a simrad plug/pigtail, I cut the garmin off(left 300mm wire on) all I did was use bullet connectors with di-electric grease and taped them up. Nowt special, I was going to replace the bullets with a waterproof plug but never got around to it. It has never missed a beat.

Anyone need a garmin pig tail pm me, it's free you pay the postage.
Paul

 

I've cut mine so I didn't have to make a huge hole in my kayak for the connector. It's soldered and heat shrinked and works fine. If in doubt just use an outdoor waterproof junction box from Screwfix.

Jon

 

Make the joins waterproof and you'll be fine.