Waterproofing a cat5e/6 repair
As others have noted - repair is not a good choice if re-running is not a very unacceptable option.
Water will have run back inside the sheath and drying out would be hard and time consuming. Water may affect the characteristics - but maybe not enough to matter to you. ie it MAY be possible to operate the cable with a small amount of water inside the sheath. Or not.
Joints can be made using good soldering techniques and adhesive lined heatshrink sleeving - not just standard heatshrink where water will travel along under the sheath with ease.
Then an over-layer of heat-shrink with adhesive over all wires and existing sheath.
Obviously the order of adding the unshrunk heatshrink sleeves needs thinking about in advance :-).
If you can bring the joint to an above ground housing so much the better. Above the water table is better than below and above ground better again.
Neutral cure good quality silicone rubber in addition to the over sleeve of heatshrink will improve chances of success. Adding some moisture absorbing compound inside the seal MAY help but you don't want something that can re-release water.
Corrosion is caused by metal-water-oxygen reaction. Water vapour will in time penetrate any flexible material known to a significant extent. Glass and metals less so - but they are not useful here. Eliminating liquid water so that there is only water vapour at the joint surface very very greatly eliminates corrosion rate. This requires a low water solubility compound around the wires and a void free contact between sealing compound and metal - that is in part what the heatshrink adhesive fill is for. Other compounds could be applied over the join inside the heatshrink adhesive seal. Sylgard 184 - intended for PV panel encapsulation MAY work. As may Dow Corning (maybe now rebranded) 1-2577 conformal coating.
Underwater cable ideally requires the use of underwater cable.
Cable &quot;severed by something&quot; ideally needs a something-proof cover - a conduit makes sense or failing that a dig proof layer - rock / gravel / slab or whatever