How to repair frayed cable

[reefgirl189]

I'm having some trouble repairing a cord from a used piece of equipment I bought from another canreefer.

The cord from the tunze pump unfortunately will sit partway in the water of my DT. The protective sheathing is cracked, exposing all the wires inside. Luckily the internal wires still have their sheathing.

Originally my plan was to get some good heat shrink and use that but I can't find any that will fit over the end of the cord and shrink enough to enclose the exposed wires to the point of being considered waterproof.

I'm nervous of using electrical tape to do the job as it is going to be underwater.

Does anyone have any ideas on what could work? I'm down to my last brilliant idea of using silicone to seal it.