Tapping undersea cables is one of the most critical missions modern day submarines conduct. So in short: every country that can get a sub that far north.
What entities of strategic importance are still sending unencrypted data over third party fiber?
Normal people do so all the time and their communications are of strategic importance as well.
What kind of “normal people communication” specifically are we referring to that isn’t encrypted on the wire?
Metadata, which isn't encrypted, and is still the most powerful part of Prism/Xkeyscore, because most people don't try to understand it.