I used to make underwater cables years ago.
One of the cable types we used was "sewer exploration cable".
It had a really tough jacket.
I don't have all the the details now, but...
We used to sandpaper the jacket and clean it.
I think we even applied some liquid coating then???
We had this rubber tape that wasn't adhesive, more like gummy, maybe 1/16" thick.
You'd wrap the tape around the splice.
We had aluminum molds and a heater.
You'd place the splice in the mold.
There was a vent at the top where you'd screw in a cylinder that you had stuffed with some rolled up rubber tape.
A screw at the top would compress the (now slightly liquid) tape into the mold.
You'd end up with a nice solid rubber lozenge around the splice.
Finally, when splicing coax, make sure you have some slack for differential shield/inner stretching.