if the cable would have to care about the conductivity of saltwater then you would have a problem
Actually not entirely true at least theoretically: surrounding wire carrying AC with a conductive medium will increase losses due to induced currents. On the other hand if the cable is balanced then the induced currents will mostly cancel.
Whether this is a real concern or not at the distances and frequencies in question I do not know.
Be careful of Google searches on this since you will find all sorts of (mis)information from the 'golden ears' audiophiles who can hear the difference in $500 vs $5000 speaker cables, although usually not in double blind tests.
One other advantage of long distance power transmission by DC is that it eliminates the need to keep the interconnected grids in phase.
As others have noted mechanically switching DC at much over 48 Volts is much more challenging due to the constant arc.