China is taking over the business of building underwater communications cables. That means China can spy on the data.

I expect that the US also spies on that data. During the cold war the US used submarines to attack snooping devices to the Soviet Union's cables. Perhaps both countries are attaching such devices now to the cables made by the other country.

One should not make an excessive fuss about state espionage against hostile states, but these countries could well be listening to you and me.

If transmission on the cable is encrypted across the cable's whole communication system, such spying would be futile, but I'd be surprised if such encryption is in use.