I was listening an interesting presentation on monday at Helsinki on cyber security of under sea fiber optics cabling. The end result was that it is possible (not always easy or cheap, but technically possible for many countries) and often not illegal on international sea to listen to the traffic on those cables.
There are around 400 undersea fiber optic cables that carry around 95 percent international telecom traffic and China has something to do with around 100 of them (technology and/or construction work).
It is also not technically or economically feasible to make those impossible to listening to, so we need to use good end-to-end encryption on our networked applications.
News article what I heard and saw (in Finnish)
Arctic Connect Project and Cyber Security Control, ARCY publication (in English)