^undersea cables are typically made up of a central fiber line, which look like human hairs in a small tube, and a shell of insulation and copper. You need power to send the signal across the ocean floor and, as we do not generate electricity underwater, all of the current needs to be sent from one end to the other to power all of the repeaters along the way.
The fiber optic cable is 17mm, but the cable itself can't be because of the power constraint (unless I'm missing something deep; feel free to correct me if so).
The cables are on sea charts so this can be avoided, but that won't stop deliberate saboteurs. The strategy for safeguarding undersea cables has been described as “security through obscurity.”
Undersea cable is different, and I suspect that nobody cares if 99.9% of the power is wasted to heat the power wire as long as enough gets to the repeaters.
Kirchhoff's law says that if your wire is insulated, ask the current that goes in one end comes out the other. The only thing is that you need to provide sufficient voltage to push it through the long wire.