[morcheeba]

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.