Materials

[picture source]

Above is a cross-section of a typical submarine communications cable. From the outside in, the materials used are:
1 - Polyethylene
2 - Mylar tape
3 - Stranded steel wires
4 - Aluminium water barrier
5 - Polycarbonate
6 - Copper or aluminium tube
7 - Petroleum jelly
8 - Optical fibers