Sea-Bird typically recommends using the Dam/Blok and EverGrip products from PMI Industries. DamBlok makes the electrical splice and EverGrip provides the strain relief on the cable. See an example of how these products can be used.
For a quick electrical splice in the field using commonly available materials, the UNOLS (University-National Oceanographic Laboratory System) website provides a procedure using hot glue and heat shrink: https://www.unols.org/document/using-hot-glueheatshrink-terminations-rowe. Numerous cycles of deployment to great depths could compromise the seal, but it may be useful for a quick fix.