There is a debate on soldering versus crimping (crimp nut, butt connector), and I've done both.
Regardless of which you choose, I always do this;
Smear liquid tape or rtv over the connection (over the solder joint or over the crimp) then slide heat shrink over the connection before the goop dries. Apply a heat gun starting in the middle and work your way to each end. This will squish the goop through the shrink and out the ends, covering the connection and surrounding area, and will result in a watertight connection that is much better than shrink alone.