Nonsense, surely you've seen a rag-hat or someone replacing 'zincs'. The zinc anodes put into the water-box ends of sea-water cooled items is exactly that. And along the hull near the shafting and propellers. Their inspected/replaced as part of every drydocking. Even the insides of ballast tanks have a few 'zincs' welded into them in key locations.
As I said before, *active* cathodic protection is something that I've only seen used on ships in the 'mothball' fleet while tied up four abreast in places like Mare-Island (now closed).
Funny, many's the time I dragged degaussing cables around submarines in dock to reduce their overall magnetic signature. Minesweepers were about the only ones I knew with active systems though. Had to learn the function of 'D', 'H' and 'T' coils just for EM3&2.
daestrom (former EMC(ss))