If you want a real water tight seal. You can solder the joins then use Heatshrink Caps. These are a small piece of heatshrink with a closed end, and a gel cartridge in the end. When you heat, it shrinks around the cable and releases the gel and seals the join. We use them in garden lighting.

Probably a bit hard to source in your area, but in the big cities you can just go into a garden light centre and ask for heatshrink caps.

Stu