It's agood idea to seal the ends of the cables if they are at all likely to get damp. One way to do this, which I've used for many years, is to buy silicon tube which is used as fuel tubing for model aeroplane engines. It can be sourced in various diameters. It is also used as exhaust pipe in the larger sizes. The large size will fit starter motor cables. It's very flexible, it takes a 'tack' to the surface it is in contact with, it's heat resistant to hundreds of degrees celsius, it's very stretchy (It can even be forced over the connector which you've accidentally forgotten to fit the sleeve to first.), it's water, fuel, oil and alcohol (?) resistant.
The way I normally use it: lightly grease the end of the cable, slide the silicon tuble over the cable outer, solder on the connector and then slide the silicon back over the soldered shank of the connector. Extending a cable by jointing is similar: Fit the sleeve, tin both ends of the cable for about 5-6mm, bring the ends into contact by overlapping, flush solder over the ends, ensure there are no sharp nibs of solder and then slide the sleeve over the joint. It will be completely water tight.
One thing to watch for though, ensure the end of the tube is clean cut because a wee notch is a stress raiser and it may travel along the sleeve. The situation where I've found this happening is where the sleeve has been stretched over a connector and it has been cut by sharp corners of the connector as it passed over.
Hope this is of help.