There's a couple of factors at play here. One is that AC suffers from capacitive losses over long distances (high power multi-megawatt underground/undersea cables are often HVDC for this and other reasons).

The other more interesting one is that the repeaters in this kind of fibre optic cable are usually powered from both ends, from completely separate electrical grids (so one side sends -5000V and the other sends +5000V, for example). This allows for some level of redundancy as well as thinner insulation. With AC, keeping the phases on both sides aligned would be impractical, as well as the inherent inefficiencies of AC transmission.

AC is only popular because it works with transformers to step up/down the voltage, and it would be more expensive to step up/down a DC signal using electronics (which usually involves converting to AC internally anyway).

AC Voltage is specified in RMS volts, which is based on the average power the AC transmits. The peak voltage (top of the sine wave) is 1.414x the RMS voltage. The insulator only cares about the peak before it breaks down, so because DC doesn't waste time at lower voltages, can transmit more power for the same insulation.

These are coax cables, just by the nature of the external physical shielding required (steel cable sheath). So, the EMF should be contained inside and not affected by the salt water. But, I'm not an expert there and could be missing something.

In addition to the other replies, I also recall hearing some time ago that the AC EM field interacted with wildlife in surprising ways (causing sharks to attack the cable, IIRC). It could be an urban legend at this point though.

unrelated but the default power supply for Central Office telecom equipment was always -48V DC (and 23" vs 19" racks)