Waterproof shrink solder connectors?

Ford motor co. recommends un-insulated crimp barrels for all wire splices. They are listed under part nos. WT-56814 for 22-18 AWG, WT-56815 for 16-18 AWG, and WT-56816 for 12-10 AWG.
Raychem environmental splices also use un-insulated barrels. both of these use adhesive lined heat shrink to seal the splice.
Raychem splices are fairly expensive, but they are the only ones most jet aircraft manufacturers approve for splicing wire.
A more economical way, is to order the barrels and heat shrink from Mouser or Digi-Key. The top picture is a Raychem and the bottom is from Mouser. both of these do an incredible job of splicing wire and the splice will more than likely outlive you.
Env splice 1A (800x600).jpg