Modern submarine cable is all fiber-optic. It's about a foot in diameter. In it they run just a few strands of fiber optic cable. It's packed in Vaseline--seriously, that's what they use. Then it's wrapped in plastic, metal and fiberglass jackets for protection.
In the cable are inline amplifiers that boost the signal.
Now, in the OLD days before submarine cable went digital, each wire pair in a submarine cable carried 16 channels of communication, as opposed to 12 channels on a terrestrial link. They did it that way because submarine cable is so expensive.