Used to use these on military projects. They are heat shrink tubes, with a bit of solder in the middle and a sealing material at both ends. You slide these on to one of your wires, twist the wires together and then slide the sleeve over the connection... then heat.

The solder melts, makes for a great soldered connection, and the sealing material at either end softens and as the sleeve closes over the wires, the sealer fills the gaps.

Look for "Immersion Resistant."