See https://en.wikipedia.org/wiki/Submarine_communications_cable... for a brief overview.

Early submarine cables were literally just wires. To overcome the electrical resistance of a long cable, they simply applied a large voltage on one end, and used sensitive instruments to pick up the tiny signal on the other end.

Interestingly, the early engineers didn't fully understand how the electromagnetic properties of a cable (capacitance and inductance) affect signal propagation over long distances. So they ended up seeing unexpected amounts of distortion that forced them to transmit much more slowly than desired. The theoretical breakthrough that enabled reliably sending signals from one side of an ocean to another was thanks to Oliver Heaviside, who was pretty much the archetype of a misunderstood crackpot/genius.

But yeah, later cables included in-line repeaters, once the technology was sufficiently well developed. Here's a description of some early vacuum-tube amplifiers that were designed for extreme reliability: http://www.ase-museoedelpro.org/Museo_Edelpro/Catalogo/tubes...

Power does diminish with distance due to I2R losses. For cables using AC, there are also reactive losses. To compensate, the cable companies boost the voltage at both ends (30-500 kV) of the cable such that the repeater at the middle of the span has enough juice to operate properly. And, yes, separate cables, or wire bundles, carry the power and ground.

The Azores

The Azores?