There are high speed SONET/SDH (common transport layer protocol ) encryptors available. I would assume that any undersea cables providers who might expect US tapping would use them. Especially if one endpoint is in Russia or China.
How many networks still run SONET at this point? Most global carriers would run OTN over DWDM at 25GHz or 50GHz spacing, with up to 200Gbps per channel. There is of course OTN encryption though