I worked on the development of the first optical fibre cables from 1980s onwards and this problem first showed up on an ATT trial off Gran Canary (around 1983) when they had left off the Glover Barrier - i.e. the cable was no longer screened. The AC ripple on the 20kV DC power feed attracted one species of shark that hunts fish by detecting the electric fields generated as they swim or struggle if they are in distress.
So very easy to fix - ATT replaced the Glover Barrier and the ripple is screened - and then the sharks ignore the cable. Fortunately the UK companies did not make the same mistake!
AND BTW cables are only ever armoured is shallow water when they cannot be buried - and when they are subject to anchors, fishing trawls, and worse - tidal abrasion on rocks. In deep water the cables either lay on the sea bed or form catenaries spanning undersea mountains, ridges.
Sadly, we live in a world of growing data and rapidly rising ignorance!