Watertight mil-spec shipboard cables are gel-filled or include expanding fillers that stop the penetration of water. These fill all the cavities in the wire, and prevent water from wicking through it. This is only to prevent compartment-to-compartment flooding through severed cables, though; all connections are always made in watertight boxes. Thankfully, you don't need to worry about this, just the jacket integrity and the connections. There should not be any water anywhere near the end of the utility's termination. It should be sealed and watertight.
Copper corrodes, even when immerse entirely in freshwater due to dissolved gasses, impurities, and simply the natural corrosive properties of H2O itself. Bottom line is you need to keep the water out, not in. Plugging the indoor end with a giant wad of RTV might be a stop-gap and get you into warmer weather, but you're going to need to replace the cable.
