Somebody in the Washington Post once quote a phrase from a sailor's prayer: "O, Lord, my ship is so small and thy ocean so large." How many thousands of miles of cable, in how many hundreds of thousands of square miles of ocean?
In 2008 and again in 2011 there were clusters of incidents involving undersea cables that led to disruption.
I can’t imagine a single ship being able to effectively monitor undersea cables.
But I can imagine a mothership deploying semi-autonomous persistent drones and sensors to detect and someday possibly even disrupt/destroy threats to the cables.
A simple CARVER matrix says the cables are far harder to defend than attack.
However, the potential attackers are limited to a very small number of sovereign states with the capability to detect/disrupt cable traffic and a larger but still small number of sovereign states with the capability of destroying cable traffic.
As described in Blind Man's Bluff, US subs regularly tapped USSR undersea cables. Divers would attach the bug and they would come back weeks or months later to pick up the tapes. But these were copper cables.
How do you bug a fiber optic cable? Solve that problem and then place an underwater drone alongside.
I read this and was worried until I pondered if critical communication was still reliant on undersea cables. A threat to the economic system is obviously an important factor.
I am no expert but I would hope that satellites are now used for defence based communication.
The bandwidth available via undersea fiber optic cables is orders of magnitude higher than via satellite. I'm sure governments have backup satellite links but their use would be restricted to top priority traffic if something catastrophic happened to the undersea cables.
Blind Man's Bluff - Book from Amazon, and also a youtube doco https://www.youtube.com/watch?v=fJ0X8ROMSUw