The adversaries tapping undersea cables are state actors operating extremely expensive kit.

The US did this in the Cold War as Operation Ivy Bells. A spy leaked this to the soviets who then retrieved a tap.

But it went on. Nowadays the US operates the USS Jimmy Carter and the Russians have a whole directorate, GUGI, operating a fleet of special submarines and surface ships.

Links: http://www.hisutton.com/Secret%20Sub%20-%20USS%20Halibut.htm...

http://www.hisutton.com/Undersea_Cables.html

I get your point but I wonder about “many”. Of the 532 cable runs, I wager that a cut cable is more impactful than traffic analysis. In USA, the specifics of eavesdropping even require that the origin of one side of the traffic is assumed or known.