It is not normally recommended to splice coax, but instead install connectors. I did it anyway to make a repair on an aerial feeder on a roof. It was successful, with negligible performance impact.

I repaired some RG-58 that had worn down to the core. First I taped the core to the same diameter as the dielectric with electrical tape (PVC). I then used aluminium foil for the shield. I taped over that again, and finished with self amalgamating tape.

The cable worked fine on 2m and 70cm, with only a small bump visible on a VNA / DTF measurement. It kept working for years until I changed it. When I did, I found the foil had partly oxidised to powder, probably from moisture that was already in the cable, but it still worked.

Note that this was not in a marine environment, and was only exposed to rain. The cable was tinned copper.