While heat-shrink or self-vulcanizing rubber tape overlaid with vinyl electrical tape is best, a splice properly
taped with good
vinyl electrical tape (3M) can be surprisingly water-tight for a surprisingly long time.
I've had outside electrical cords that I had chained together, having taped the connections, that had been used intermittently for years, sometimes sitting out for days on end in the snow or rain, that I subsequently took apart that looked like the day they came from the store. I've had roof-top antennas that had been up there for years
, exposed to all manner of weather conditions, where the connectors were found to be in like-new condition upon inspection. When I say "like new," I mean not a spot
of oxidation. Oh, and those antenna connectors were usually silver plated
What vinyl electrical tape is not
suitable for is when the repaired area is likely to be flexed or exposed to other mechanical stress. I wouldn't trust it under water, either.