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.
Jim