For it to be illegal, it would need to be in some sort of jurisdiction. Which like it or not, undersea cables usually are not.

Is it illegal to spy on communications under sea in deeply international water?

It should be a crime to communicate across any public link without encryption. I’m surprised there anything of value on these links besides flow analysis.

If you know the src and dst IPs, you can impose arbitrary firewall rules and cut down specific connections. That’s what China is already doing with their national firewall. I’m not sure if there is an additional layer of encryption at both ends that would encrypt packet headers as well.